Electrochemical capture of Lewis acid gases

ABSTRACT

Methods, apparatuses, and systems related to electrochemical capture of Lewis acid gases from fluid mixtures are generally described. Certain embodiments are related to electrochemical methods involving selectively removing a first Lewis acid gas from a fluid mixture containing multiple types of Lewis acid gases (e.g., a first Lewis acid gas and a second Lewis acid gas). Certain embodiments are related to electrochemical systems comprising certain types of electroactive species having certain redox states in which the species is capable of binding a first Lewis acid gas but for which binding with a second Lewis acid gas is thermodynamically and/or kinetically unfavorable. The methods, apparatuses, and systems described herein may be useful in carbon capture and pollution mitigation applications.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/005,250, filed Aug. 27, 2020, entitled “Electrochemical Capture ofLewis Acid Gases,” which claims priority under 35 U.S.C. § 119(e) toU.S. Provisional Application No. 62/892,975, filed Aug. 28, 2019, andentitled “Electrochemically Mediated Acid Gas Removal andConcentration,” and to U.S. Provisional Application No. 62/988,851,filed Mar. 12, 2020, and entitled “Electrochemical Capture of Lewis AcidGases,” each of which is incorporated herein by reference in itsentirety for all purposes.

TECHNICAL FIELD

Methods, apparatuses, and systems related to electrochemical capture ofLewis acid gases from fluid mixtures are generally described.

BACKGROUND

Efforts have been made to remove or separate gases from fluid mixtures.For example, over the last two decades there has been an effort tomitigate global warming by curbing anthropogenic carbon dioxide (CO₂)emission. A number of approaches, such as conventional thermal methods,have been pursued to tackle carbon dioxide capture at different stagesof its production: either post combustion capturing at power plants, orconcentrating it from the atmosphere, after which it is eitherpressurized and stored in geological formations, or it is converted tocommercially useful chemical compounds. One alternative approach iselectrochemical capture of gases using electroactive species. However,challenges may occur in removing or separating certain gases (e.g.,carbon dioxide) in fluid mixtures containing multiple different types ofgases (e.g., multiple types of Lewis acid gases).

Improved apparatuses, methods, and/or systems are desirable.

SUMMARY

Methods, apparatuses, and systems related to electrochemical capture ofLewis acid gases from fluid mixtures are generally described. Certainembodiments are related to electrochemical methods involving selectivelyremoving a first Lewis acid gas from a fluid mixture containing multipletypes of Lewis acid gases (e.g., a first Lewis acid gas and a secondLewis acid gas). Some embodiments are related to methods involvingselective Lewis acid gas removal by bonding a first Lewis acid gas and asecond Lewis acid gas to one or more reduced electroactive species and,subsequently, selectively releasing the second Lewis acid gas from theresulting complexes while releasing relatively little or none of thefirst Lewis acid gas from the complexes. Certain embodiments are relatedto electrochemical systems comprising certain types of electroactivespecies having certain redox states in which the species is capable ofbinding a first Lewis acid gas but for which binding with a second Lewisacid gas is thermodynamically and/or kinetically unfavorable. Themethods, apparatuses, and systems described herein may be useful incarbon capture and pollution mitigation applications. The subject matterof the present invention involves, in some cases, interrelated products,alternative solutions to a particular problem, and/or a plurality ofdifferent uses of one or more systems and/or articles.

In one aspect, methods are described. In some embodiments, the methodcomprises applying a potential difference across an electrochemicalcell, exposing a fluid mixture comprising a first Lewis acid gas and asecond Lewis acid gas to the electrochemical cell, and removing anamount of the first Lewis acid gas from the fluid mixture during and/orafter the applying the potential difference, wherein the method involvesremoving from the fluid mixture essentially none or less than or equalto 10% of the second Lewis acid gas present in the fluid mixture by molepercent.

In some embodiments, a method, comprises exposing a fluid mixturecomprising a first Lewis acid gas and a second Lewis acid gas to one ormore electroactive species in a reduced state; bonding an amount of thefirst Lewis acid gas to a first portion of the one or more electroactivespecies in the reduced state to form first Lewis acid gas-electroactivespecies complexes; bonding an amount of the second Lewis acid gas to asecond portion of the one or more electroactive species in the reducedstate to form second Lewis acid gas-electroactive species complexes; andoxidizing at least some of the second Lewis acid gas-electroactivespecies complexes such that an amount of the second Lewis acid gas isreleased from the second Lewis acid gas-electroactive species complexeswhile releasing essentially none of the first Lewis acid gas from thefirst Lewis acid gas-electroactive species complexes or releasing anamount of the first Lewis acid gas from the first Lewis acidgas-electroactive species complexes that is less than or equal to 10% ofthe first Lewis acid gas-electroactive species complexes by molepercent.

In another aspect, electrochemical apparatuses are described. In someembodiments, the electrochemical apparatus comprises a chambercomprising a negative electrode in electronic communication with anelectroactive species, the chamber constructed to receive a fluidmixture, wherein, in at least one conductive medium, the electroactivespecies has an oxidized state and at least one reduced state in whichthe electroactive species is capable of bonding with a first Lewis acidgas, but for which a reaction with a second Lewis acid gas comprisingone or more species chosen from carbon dioxide, nitrogen oxides, R₃B, orR₂S is thermodynamically and/or kinetically unfavorable at at least onetemperature, wherein each R is independently H, branched or unbranchedC1-C8 alkyl, aryl, cyclyl, heteroaryl, or heterocyclyl. In someembodiments, the electroactive species has at least one reduced state inwhich the electroactive species is capable of bonding with a first Lewisacid gas, but for which a reaction with a second Lewis acid gascomprising one or more species chosen from carbon dioxide, nitrogenoxides, a borane, or hydrogen sulfide (H₂S) is thermodynamically and/orkinetically unfavorable at at least one temperature greater than orequal to 223 K and less than or equal to 573 K.

Other advantages and novel features of the present invention will becomeapparent from the following detailed description of various non-limitingembodiments of the invention when considered in conjunction with theaccompanying figures. In cases where the present specification and adocument incorporated by reference include conflicting and/orinconsistent disclosure, the present specification shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale. In the figures,each identical or nearly identical component illustrated is typicallyrepresented by a single numeral. For purposes of clarity, not everycomponent is labeled in every figure, nor is every component of eachembodiment of the invention shown where illustration is not necessary toallow those of ordinary skill in the art to understand the invention. Inthe figures:

The figures are described in-line with the text below.

FIGS. 1A-1B are schematic illustrations of an exemplary process forremoving first Lewis acid gas from a fluid mixture comprising the firstLewis acid gas and a second Lewis acid gas, according to one or moreembodiments;

FIG. 2 is a cross-sectional schematic diagram of an electrochemicalapparatus comprising a chamber constructed to receive a fluid mixture,and a negative electrode, according to one or more embodiments;

FIG. 3 is a schematic illustration of an exemplary process for removingfirst Lewis acid gas from a fluid mixture comprising the first Lewisacid gas and a second Lewis acid gas, according to one or moreembodiments;

FIG. 4 is a cross-sectional schematic diagram of a flow apparatuscomprising a first electrochemical cell and a second electrochemicalcell, according to one or more embodiments;

FIG. 5 shows a side-view schematic diagram of an exemplaryelectrochemical cell comprising a negative electrode, a positiveelectrode, and a separator, according to one or more embodiments;

FIG. 6 shows an exploded schematic diagram of an exemplaryelectrochemical cell, according to one or more embodiments;

FIG. 7A shows a schematic drawing of an exemplary system performing agas separation process, according to one or more embodiments;

FIG. 7B shows a schematic drawing of an exemplary system comprising flowfields, performing a gas separation process, according to one or moreembodiments;

FIGS. 7C-7E show side view schematic illustrations of exemplary flowfield channel patterns, according to one or more embodiments;

FIG. 8A shows a schematic drawing of an exemplary system comprising aplurality of electrochemical cells performing a gas separation process,according to one or more embodiments;

FIG. 8B shows a schematic drawing of an exemplary system comprising aplurality of electrochemical cells electrically connected in parallelperforming a gas separation process, according to one or moreembodiments;

FIG. 8C shows a schematic drawing of an exemplary system comprising aplurality of electrochemical cells electrically connected in seriesperforming a gas separation process, according to one or moreembodiments;

FIG. 9 shows a schematic drawing of an exemplary system comprising aplurality of electrochemical cells electrically connected in series andone or more electrically conductive materials between electrochemicalcells, performing a gas separation process, according to one or moreembodiments;

FIG. 10A shows cyclic voltammetry of 1,4-naphthoquinone (p-NQ,alternatively labeled as NQ) in a dry N,N-dimethylformamide solutioncontaining 0.1 M tetra-n-butylammonium hexafluorophosphate ([nBu₄][PF₆])saturated with either N₂, CO₂, or SO₂;

FIG. 10B shows cyclic voltammetry of by 2,3-dicyano-1,4-naphthoquinone(DCNQ) in a dry N,N-dimethylformamide solution containing 0.1 M[nBu₄][PF₆] saturated with either N₂, CO₂, or SO₂;

FIG. 11A shows thermogravimetric analyses (TGA) of DCNQ²⁻, NQ^(⋅−) andNQ^(⋅2−) under 1% SO₂;

FIG. 11B shows TGA analyses of DCNQ²⁻, NQ^(⋅−) and NQ²⁻ in N₂;

FIG. 11C shows TGA measurements of SO₂ uptake by NQ²⁻ at differenttemperatures;

FIG. 11D shows TGA measurements of SO₂ uptake release by NQ²⁻ atdifferent temperatures;

FIG. 11E shows TGA measurements of SO₂ uptake by DCNQ²⁻ at differenttemperatures;

FIG. 11F shows TGA measurements of SO₂ release by DCNQ²⁻ at differenttemperatures;

FIG. 12A shows TGA measurements of capture of CO₂ with reduced DCNQunder 100% CO₂ at 30° C.;

FIG. 12B shows TGA measurements showing that CO₂ is released when thereactant is removed;

FIGS. 13A-13C show the computed geometry changes of DCNQ upon reduction;

FIG. 14A shows electrostatic potential (ESP) maps of2,3-dicyano-1,4-naphthoquinone (top) and 1,4-naphthoquinone (bottom) intheir respective neutral states;

FIG. 14B shows ESP maps of 2,3-dicyano-1,4-naphthoquinone (top) and1,4-naphthoquinone (bottom) in their respective semiquinone states;

FIG. 14C shows ESP maps of 2,3-dicyano-1,4-naphthoquinone (top) and1,4-naphthoquinone (bottom) in their respective dianion states;

FIGS. 15A-15D shows calculated geometries and ESP maps of CO₂ and SO₂,respectively;

FIG. 16 shows a schematic diagram of the gas separation experiment,according to one or more embodiments; and

FIGS. 17A-17B shows a plot of a ratio of outlet gas concentration toinlet gas concentration vs. time for the physisorption and chemisorptionexperiments, according to one or more embodiments.

DETAILED DESCRIPTION

Methods, apparatuses, and systems related to electrochemical capture ofLewis acid gases from fluid mixtures are generally described. Certainembodiments are related to electrochemical methods involving selectivelyremoving a first Lewis acid gas (e.g., sulfur dioxide) from a fluidmixture containing multiple types of Lewis acid gases (e.g., a firstLewis acid gas and a second Lewis acid gas (e.g., carbon dioxide)). Someembodiments are related to methods involving selective Lewis acid gasremoval by bonding a first Lewis acid gas (e.g., sulfur dioxide) and asecond Lewis acid gas (e.g., carbon dioxide) to one or more reducedelectroactive species and, subsequently, selectively releasing thesecond Lewis acid gas (e.g., via oxidation of a second Lewis acidgas-electroactive species complex) from the resulting complexes whilereleasing relatively little or none of the first Lewis acid gas from thecomplexes. Certain embodiments are related to electrochemical systemscomprising certain types of electroactive species having certain redoxstates in which the species is capable of binding a first Lewis acid gasbut for which binding with a second Lewis acid gas is thermodynamicallyand/or kinetically unfavorable. The methods, apparatuses, and systemsdescribed herein may be useful in carbon capture and pollutionmitigation applications.

Removal and/or separation of Lewis acid gases in fluid mixtures is animportant process in a number of applications, including industry andpower generation. As an example, sulfur dioxide (SO₂) emissions areconventionally curtailed in industrial applications via flue-gasdesulfurization (FGD), which relies on large absorber contact towers(scrubbers) which have large footprints and a large balance of plant.Moreover, many other applications exist where it can be desirable toremove combustion exhaust or other industrial gas streams where Lewisacid gases such as SO₂, either for avoiding complications withdownstream processes or for other reasons, such as pollution mitigation.For example, the international Maritime Organization (IMO) has imposed asulfur cap that went in effect in January 2020. Many ships are currentlybeing retrofitted with convention scrubbers, but small vessels cannotaccommodate such chemical plants on-board. Therefore, there is a needfor compact and efficient Lewis acid gas capture systems.Electrochemically-mediated capture of gases may be one route towardcapturing Lewis acid gases (e.g., by electrochemically generating activestates of electroactive species that can bind the targeted gases).However, as a complicating factor, many fluid mixtures (e.g., gaseffluents) comprise multiple Lewis acid gas species, and it can bedesirable to selectively remove a first Lewis acid gas while removingessentially none or relatively little of the other Lewis acid gases. Asone example, combustion products often comprise carbon dioxide andsulfur-containing gases such as SO₂. It has been realized in the contextof the present disclosure that certain existing electroactive species inelectrochemical systems may react with both the first Lewis acid gasesand the other Lewis acid gases, which can be problematic. For carboncapture systems designed to remove carbon dioxide from the gas mixture,the presence of sulfur dioxide can pose problems for efficiency andcapacity because the sulfur dioxide may compete with the carbon dioxidefor binding to the electroactive species. It has been discovered in thecontext of the present disclosure that certain methods and systems canbe used to selectively bind certain Lewis acid gases over others (e.g.,via judicious choice of electroactive species and/or operatingconditions).

In one aspect, methods are described. Some embodiments involve methodsfor electrochemical partial or complete removal and/or separation ofLewis acid gases in fluid. Some embodiments comprise applying apotential difference across an electrochemical cell and exposing a fluidmixture comprising a first Lewis acid gas and a second Lewis acid gas tothe electrochemical cell. FIGS. 1A-1B depict one such embodiment, wherefluid mixture 101 comprising first Lewis acid gas 102 and second Lewisacid gas 104 is exposed to electrochemical cell 100. The term“electrochemical cell” is intended to include apparatuses that meetthese criteria even where the behavior of the cell could arguably becharacterized as more pseudocapacitive than Faradaic and thus mightotherwise be referred to as a type of capacitor.

Applying a potential difference across the electrochemical cell maycause an amount of the first Lewis acid gas to be removed from the fluidmixture during and/or after application of the potential difference. Forexample, referring again to FIGS. 1A-1B, in the absence of the potentialdifference, electrochemical cell 100 may be in electronic communicationwith electroactive species in an oxidized state Ox that does not bindwith the first Lewis acid gas (FIG. 1A). Applying a potential differenceacross electrochemical cell 100 may convert the electroactive speciesinto a reduced state R that reacts (e.g., via binding) with first Lewisacid gas 102 (FIG. 1B), according to some embodiments. It should beunderstood that while FIGS. 1A-1B show fluid mixture 101 exposed toelectrochemical cell 100 with the electroactive species in oxidizedstate Ox, some embodiments comprise applying the potential differenceacross electrochemical cell 100 prior to exposing it to fluid mixture101, such that at least one reduced state (e.g., R) of the electroactivespecies is generated prior to exposure to fluid mixture 101.

In some embodiments, the method involves removing from the fluid mixtureessentially none or relatively little of the second Lewis acid gaspresent in the fluid mixture by mole percent. For example, referringagain to FIG. 1B, essentially none of second Lewis acid gas 104 isremoved from fluid mixture 101 upon application of the potentialdifference across electrochemical cell 100 (e.g., removed by reactingwith reduced state R of the electroactive species generated by theapplication of the potential difference, in contrast to first Lewis acidgas 102). In some instances, removing essentially none or relativelylittle of a second Lewis acid gas from a fluid mixture comprising afirst Lewis acid gas and the second Lewis acid gas may be beneficial ifit is desired to produce a fluid mixture relatively free of the firstLewis acid gas. As one non-limiting example, a fluid mixture maycomprise carbon dioxide (CO₂) and sulfur dioxide (SO₂), but it isdesired for the fluid mixture to contain only CO₂ (e.g., for adownstream carbon capture process). Therefore, in some embodiments,applying a potential difference to an electrochemical cell and exposingthe fluid mixture to the electrochemical cell may result in removal ofan amount (or all) of the SO₂ while removing essentially none orrelatively little of the CO₂. The reduced concentration of SO₂ in thefluid mixture after the performance of such a method may increase theefficiency and/or capacity of a downstream process involving the fluidmixture.

Removing an amount of the first Lewis acid while removing essentiallynone or relatively little of the second Lewis acid may be accomplishedaccording any of the variety of techniques described in the presentdisclosure (alone or in combination). For example, removing an amount ofthe first Lewis acid gas while removing essentially none or relativelylittle of the second Lewis acid may comprise exposing a fluid mixturecomprising the first Lewis acid gas and second Lewis acid gas toconditions configured such that the first Lewis acid gas bonds with anelectroactive species but the second Lewis acid gas does not bond withthe electroactive species (e.g., due to thermodynamic unfavorability orkinetic reasons). In some embodiments, the conditions are configuredsuch that both the first Lewis acid gas and second Lewis acid gas canbond with an electroactive species, but the first Lewis acid gas has agreater affinity (as measured by an equilibrium binding constant underthe operative conditions) for the electroactive species than does thesecond Lewis acid gas. Some such configurations may result in the secondLewis acid gas (e.g., carbon dioxide) bonding to an electroactivespecies reversibly and the first Lewis acid gas bonding to theelectroactive species irreversibly such that the first Lewis acid gasoutcompetes and/or displaces the second Lewis acid gas. A net result ofsuch reactivity is an amount of the first Lewis acid gas being removedfrom the fluid mixture while relatively little or none of the firstLewis acid gas is removed, despite formation of first Lewis acidgas-electroactive species complexes during at least some of the overallprocess. As yet another format, removal of the first Lewis acid gas mayoccur by bonding the first Lewis acid gas and the second Lewis acid gas(e.g., carbon dioxide) to one or more reduced electroactive species and,subsequently, selectively releasing the second Lewis acid gas (e.g., viaoxidation of a second Lewis acid gas-electroactive species complex or achange in temperature) from the complexes while releasing relativelylittle or none of the first Lewis acid gas from the complexes.

In some embodiments, the method involves removing an amount of the firstLewis acid from the fluid mixture and removing from the fluid mixtureless than or equal to 10%, less than or equal to 5%, less than or equalto 1%, less than or equal to 0.5%, less than or equal to 0.1%, less thanor equal to 0.05%, less than or equal to 0.01%, less than or equal to0.001%, and/or as little as 0.0001%, as little as 0.00001% or less ofthe second Lewis acid gas present in the fluid mixture by mole percent.In some embodiments, the method involves removing from the fluid mixtureless than or equal to 10%, less than or equal to 5%, less than or equalto 1%, less than or equal to 0.5%, less than or equal to 0.1%, and/or aslow as 0.05%, as low as 0.01%, as low as 0.001%, and/or as little as0.0001%, as little as 0.00001% or less of the second Lewis acid gaspresent in the fluid mixture by volume percent. In some embodiments,essentially none (e.g., none or a negligible amount with respect to thepurpose of the fluid mixture such as carbon capture or purified gasproduction) of the second Lewis acid gas is removed during theperformance of the method.

The potential difference applied across the electrochemical cell may beperformed in a charge mode. In the charge mode, a redox half reactiontakes place at the negative electrode in which the electroactive speciesof the negative electrode is reduced. The potential difference appliedacross the electrochemical cell, during a charge mode, may have aparticular voltage. The potential difference applied across theelectrochemical cell may depend, for example, on the standard reductionpotential for the generation of at least one reduced state of theelectroactive species, as well as the standard reduction potential forthe interconversion between a reduced state and an oxidized state of asecond electroactive species, when present. In some embodiments, thepotential difference is at least 0 V, at least 0.1 V, at least 0.2 V, atleast 0.5 V, at least 0.8 V at least 1.0 V, at least 1.5 V, or higher.In some embodiments, the potential difference is less than or equal to2.0 V, than or equal to 1.5 V, than or equal to 1.0 V, less than orequal to 0.5 V, or less. Combinations of these voltages are alsopossible. For example, in some embodiments, the potential differenceapplied across the electrochemical cell is at least 0.5 V and less thanor equal to 2.0 V. Other values are also possible.

The potential difference applied across the electrochemical cell may beperformed in a discharge mode. In the discharge mode, a redox half takesplace at the negative electrode in which the electroactive species ofthe negative electrode is oxidized. The potential difference across theelectrochemical cell, during a discharge mode, may have a particularvoltage. For example, in some embodiments, the potential difference maybe less than 0 V, less than or equal to −0.5 V, less than or equal to−1.0 V, or less than or equal to −1.5 V. In some embodiments, thepotential difference may be at least −2.0 V, at least −1.5 V, at least−1.0 V or at least −0.5 V. Combinations of these voltages are alsopossible, for example, at least −2.0 V and less than or equal to −0.5 V.Other values are also possible.

The fluid mixture that is exposed to the electrochemical cell may comein any of a variety of forms and compositions. In some embodiments, thefluid mixture is a gas mixture. For example, fluid mixture 101 in FIGS.1A-1B is a gas mixture comprising first Lewis acid gas 102 and secondLewis acid gas 104 upon exposure to electrochemical cell 100, inaccordance with some embodiments. In some embodiments, the fluid mixtureis a liquid mixture. For example, fluid mixture 101 in FIGS. 1A-1B is aliquid mixture comprising a liquid (e.g., solvent) in which first Lewisacid gas 102 and second Lewis acid gas are present (e.g., dissolved),according to some embodiments. The liquid may be any of a variety ofliquids, such as water or an organic liquid (e.g.,N,N-dimethylformamide, liquid quinones), an ionic liquid, a eutecticmixture of organic material that are liquids at certain combinations,and combinations thereof. One example of liquid quinones that may besuitable for the methods and systems herein is a liquid mixture ofbenzoquinone and a second quinone such as a naphthoquinone as isdescribed in Shimizu A, Takenaka K, Handa N, Nokami T, Itoh T, Yoshida JI. Liquid Quinones for Solvent-Free Redox Flow Batteries. AdvancedMaterials. 2017 November; 29(41):1606592, which is incorporated byreference herein for all purposes. In some embodiments, the liquid ofthe fluid mixture comprises a carbonate ester. For example, in someembodiments, the liquid comprises dimethyl carbonate, diethyl carbonate,ethyl-methyl carbonate, ethylene carbonate, propylene carbonate, orcombinations thereof.

As mentioned above, the fluid mixture may comprise a first Lewis acidgas. A Lewis acid gas generally refers to a gaseous species able toaccept an electron pair from an electron pair donor (e.g., by having anempty orbital energetically accessible to the electron pair of thedonor). In some instances, the pK_(a) of a Lewis acid gas is lower thanthat of an electroactive species of the electrode, when present, in oneor more of its reduced states. For example, in some instances theelectroactive species comprises an optionally-substituted quinone havinga semiquinone reduced state with a pK_(a), and gases having a lowerpK_(a) than that semiquinone would be considered a Lewis acid gas inthose instances. In some embodiments, the first Lewis acid gas is a gaschosen from sulfur dioxide (SO₂), sulfur oxides (SO_(x)), nitrogenoxides (NO_(x)), R₂S, carbonyl sulfide (COS), R₃B, boron trifluoride(BF₃), or a combination thereof, wherein each R is independently H,branched or unbranched C1-C8 alkyl, aryl, cyclyl, heteroaryl, orheterocyclyl. In some embodiments, R₂S is hydrogen sulfide (H₂S). Insome embodiments, R₃B is a borane. One example of a borane is BH₃. Forexample, in some instances the electroactive species comprises anoptionally-substituted quinone having a semiquinone reduced state with apK_(a), and gases having a lower pK_(a) than that semiquinone would beconsidered a Lewis acid gas in those instances. In some embodiments, thefirst Lewis acid gas is a gas chosen from sulfur dioxide (SO₂), sulfuroxides (SO_(x)), nitrogen oxides (NO_(x)), R₂S, carbonyl sulfide (COS),R₃B, boron trifluoride (BF₃), or a combination thereof, wherein each Ris independently H, branched or unbranched C1-C8 alkyl, aryl, cyclyl,heteroaryl, or heterocyclyl. In some embodiments, the first Lewis acidgas is a gas chosen from sulfur dioxide (SO₂), sulfur oxides (SO_(x)),nitrogen oxides (NO_(x)), hydrogen sulfide (H₂S), carbonyl sulfide(COS), borane (BH₃), boron trifluoride (BF₃), or a combination thereof.It should be understood that in this context, the first Lewis acid gasbeing a combination of two or more species refers to a mixturecontaining each of the two or more species, not a chemical productformed (e.g., addition product) by a reaction between the two or morespecies. One of ordinary skill in the art, with the benefit of thisdisclosure would understand applicable SO_(X) and NO_(x) Lewis acidgases, and that the “x” in these formulae refer to a variablestoichiometric coefficient. In some embodiments, the first Lewis acid isa species for which removal is desirable. For example, in certain carboncapture applications, sulfur-containing gases such as SO₂ may be presentin fluid streams (e.g., gas effluent). The sulfur-containing gases mayinterfere with carbon capture methods (e.g., by competing with adsorbentmaterials). Therefore, removing the sulfur-containing species from thefluid mixture may improve the carbon capture process.

In some embodiments, the concentration of the first Lewis acid gas inthe fluid mixture (e.g., prior to the application of the potentialdifference) is relatively high. In some embodiments, the concentrationof the first Lewis acid gas in the fluid mixture (e.g., prior to theapplication of the potential difference) is greater than or equal to0.00001 mole percent (mol %), greater than or equal to 0.0001 mol %,greater than or equal to 0.001 mol %, greater than or equal to 0.01 molepercent (mol %), greater than or equal to 0.1 mol %, greater than orequal to 0.5 mol %, greater than or equal to 1 mol %, greater than orequal to 5 mol %, greater than or equal to 10 mol %, greater than orequal to 25 mol %, greater than or equal to 50 mol %, greater than orequal to 75 mol %, greater than or equal to 90 mol %, or greater. Insome embodiments, the concentration of the first Lewis acid gas in thefluid mixture (e.g., prior to the application of the potentialdifference) is less than or equal to 99 mol %, less than or equal to 95mol %, less than or equal to 90 mol %, less than or equal to 75 mol %,less than or equal to 50 mol %, less than or equal to 25 mol %, lessthan or equal to 10 mol %, less than or equal to 5 mol %, less than orequal to 2 mol %, less than or equal to 1 mol %, or less. Combinations(e.g., greater than or equal to 0.01 mol % and less than or equal to 99mol %) are possible. Another possible combination is greater than orequal to 0.00001 mol % and less than or equal to 99 mol %.

In some embodiments, the concentration of the first Lewis acid gas inthe fluid mixture (e.g., prior to the application of the potentialdifference) is greater than or equal to 0.01 volume percent (vol %),greater than or equal to 0.1 vol %, greater than or equal to 0.5 vol %,greater than or equal to 1 vol %, greater than or equal to 5 vol %,greater than or equal to 10 vol %, greater than or equal to 25 vol %,greater than or equal to 50 vol %, greater than or equal to 75 vol %,greater than or equal to 90 vol %, or greater. In some embodiments, theconcentration of the first Lewis acid gas in the fluid mixture (e.g.,prior to the application of the potential difference) is less than orequal to 99 vol %, less than or equal to 95 vol %, less than or equal to90 vol %, less than or equal to 75 vol %, less than or equal to 50 vol%, less than or equal to 25 vol %, less than or equal to 10 vol %, lessthan or equal to 10 vol %, less than or equal to 5 vol %, less than orequal to 2 vol %, less than or equal to 1 vol %, or less. Combinations(e.g., greater than or equal to 0.01 vol % and less than or equal to 99vol %) are possible.

In some embodiments, the fluid mixture comprises a second Lewis acidgas. In some embodiments, the second Lewis acid gas comprises one ormore species chosen from carbon dioxide, nitrogen oxides, R₃B, or R₂S,wherein each R is independently H, branched or unbranched C1-C8 alkyl,aryl, cyclyl, heteroaryl, or heterocyclyl. In some embodiments, thesecond Lewis acid gas comprises one or more species chosen from carbondioxide, nitrogen oxides, a borane, or H₂S. As mentioned above, in someembodiments the second Lewis acid gas is carbon dioxide and the fluidmixture is desired to undergo a carbon capture process or a purificationprocess (to produce substantially pure carbon dioxide, e.g., for carbonsequestration). In some embodiments, R₂S is hydrogen sulfide (H₂S). Themethods described herein involving removing an amount of the first Lewisacid gas while removing essentially none or relatively little of any ofthe second Lewis acid gas (e.g., carbon dioxide) present in the fluidmixture may be beneficial for some such applications. It should beunderstood that in some embodiments, the fluid mixture comprises thefirst Lewis acid gas (e.g., SO₂) and two or more other gaseous Lewisacid species (e.g., CO₂ and NO₃), and it is desired to remove an amountof the SO₂ while removing essentially none or relatively little of thetwo or more other gaseous Lewis acid species (e.g., CO₂ and NO₃).Another example of a Lewis acid gas that may be included in the secondLewis acid gas is NO₂. In such embodiments, the second Lewis acid gas isconsidered to be the combination (as a mixture) of the two or more otherLewis acid gases (e.g., CO₂ and the NO₃). A further downstream step inwhich an amount of the one of the two or more other Lewis acid gases(e.g., NO₃) is removed (e.g., electrochemically) from the product fluidstream while removing essentially none or relatively little of the otherof the two or more other Lewis acid gases (e.g., CO₂) may then beperformed. In some instances, the methods described herein involveremoval of a sulfur-containing Lewis acid gas while removing essentiallynone or relatively little of a second, different sulfur-containing Lewisacid gas. For instance, in some embodiments the first Lewis acid gas isSO₂ and the second Lewis acid gas is H₂S, and the method comprisesremoving an amount of the SO₂ while removing essentially none orrelatively little H₂S from the fluid mixture. As yet another example, insome instances the methods described herein involve removal of a firstborane while removing essentially none or relatively little of a second,different borane.

For instance, in some embodiments the first Lewis acid gas is BH₃ andthe second Lewis acid gas is BF₃, and the method comprises removing anamount of the BH₃ while removing essentially none or relatively littleBF₃ from the fluid mixture.

In some embodiments, the concentration of the second Lewis acid gas inthe fluid mixture (e.g., prior to the application of the potentialdifference) is relatively high. In some embodiments, the concentrationof the second Lewis acid gas in the fluid mixture (e.g., prior to theapplication of the potential difference) is greater than or equal to0.01 mole percent (mol %), greater than or equal to 0.1 mol %, greaterthan or equal to 0.5 mol %, greater than or equal to 1 mol %, greaterthan or equal to 5 mol %, greater than or equal to 10 mol %, greaterthan or equal to 25 mol %, greater than or equal to 50 mol %, greaterthan or equal to 75 mol %, greater than or equal to 90 mol %, orgreater. In some embodiments, the concentration of the second Lewis acidgas in the fluid mixture (e.g., prior to the application of thepotential difference) is less than or equal to 99 mol %, less than orequal to 95 mol %, less than or equal to 90 mol %, less than or equal to75 mol %, less than or equal to 50 mol %, less than or equal to 25 mol%, less than or equal to 10 mol %, less than or equal to 5 mol %, lessthan or equal to 2 mol %, less than or equal to 1 mol %, or less.Combinations (e.g., greater than or equal to 0.01 mol % and less than orequal to 99 mol %) are possible.

In some embodiments, the concentration of the second Lewis acid gas inthe fluid mixture (e.g., prior to the application of the potentialdifference) is greater than or equal to 0.01 volume percent (vol %),greater than or equal to 0.1 vol %, greater than or equal to 0.5 vol %,greater than or equal to 1 vol %, greater than or equal to 5 vol %,greater than or equal to 10 vol %, greater than or equal to 25 vol %,greater than or equal to 50 vol %, greater than or equal to 75 vol %,greater than or equal to 90 vol %, or greater. In some embodiments, theconcentration of the second Lewis acid gas in the fluid mixture (e.g.,prior to the application of the potential difference) is less than orequal to 99 vol %, less than or equal to 95 vol %, less than or equal to90 vol %, less than or equal to 75 vol %, less than or equal to 50 vol%, less than or equal to 25 vol %, less than or equal to 10 vol %, lessthan or equal to 10 vol %, less than or equal to 5 vol %, less than orequal to 2 vol %, less than or equal to 1 vol %, or less. Combinations(e.g., greater than or equal to 0.01 vol % and less than or equal to 99vol %) are possible.

In some embodiments, a relatively large amount of the first Lewis acidgas is removed from the fluid mixture during the processes describedherein. Removing a relatively large amount of the first Lewis acid gasmay, in some cases, be beneficial for any of a variety of applications,such as capturing gases that may be deleterious if released into theatmosphere for environmental reasons, or deleterious to a downstreamprocess for the fluid mixture (e.g., carbon capture). In someembodiments the amount of first Lewis acid gas in a treated fluidmixture (e.g., a fluid mixture from which an amount of the first Lewisacid gas is removed upon being exposed to the electrochemical cell) isless than or equal to 50%, less than or equal to 25%, less than or equalto 10%, less than or equal to 5%, less than or equal to 2%, less than orequal to 1%, less than or equal to 0.5%, less than or equal to 0.1% orless of the amount (in volume percent) of the first Lewis acid gas inthe original fluid mixture prior to treatment (e.g., the amount of thetarget in the fluid mixture prior to being exposed to electrochemicalcell). In some embodiments, the amount of first Lewis acid gas in atreated fluid mixture is greater than or equal to 0.001%, greater than0.005%, greater than or equal to 0.01%, greater than or equal to 0.05%,greater than or equal to 0.1%, greater than or equal to 0.5%, greaterthan or equal to 1%, greater than or equal to 2%, greater than or equalto 5%, or greater of the amount (in volume percent) of the first Lewisacid gas in the original fluid mixture prior to treatment.

In some embodiments the amount of first Lewis acid gas in a treatedfluid mixture (e.g., a fluid mixture from which an amount of the firstLewis acid gas is removed upon being exposed to the electrochemicalcell) is less than or equal to 50%, less than or equal to 25%, less thanor equal to 10%, less than or equal to 5%, less than or equal to 2%,less than or equal to 1%, less than or equal to 0.5%, less than or equalto 0.1% or less of the amount (in mole percent) of the first Lewis acidgas in the original fluid mixture prior to separation (e.g., the amountof the target in the fluid mixture prior to being exposed toelectrochemical cell). In some embodiments, the amount of first Lewisacid gas in a treated fluid mixture is greater than or equal to 0.001%,greater than 0.005%, greater than or equal to 0.01%, greater than orequal to 0.05%, greater than or equal to 0.1%, greater than or equal to0.5%, greater than or equal to 1%, greater than or equal to 2%, greaterthan or equal to 5%, or greater of the amount (in mole percent) of thefirst Lewis acid gas in the original fluid mixture prior to treatment.

As mentioned above, certain electroactive species may be used during themethods described herein for removing an amount of the first Lewis acidgas while removing essentially none or relatively little of the secondLewis acid gas. As used herein, an electroactive species generallyrefers to an agent (e.g., chemical entity) which undergoes oxidation orreduction upon exposure to an electrical potential in an electrochemicalcell. It should be understood, however, that while electroactive speciesmay undergo electrical potential-induced oxidation and reductionreactions, it may also be possible to induce changes in oxidation statechemically (e.g., via exposure to a chemical reductant or chemicaloxidant in solution or at a surface). In some, but not necessarily allembodiments, an electrode comprises the electroactive species. It shouldbe understood that when an electrode comprises an electroactive species,the electroactive species may be located at a surface of the electrode,in at least a portion of the interior of the electrode (e.g., in poresof the electrode), or both. For example, referring to FIG. 2 , in someembodiments, electrochemical cell 100 comprises negative electrode 110,and negative electrode 110 comprises an electroactive species. Theelectroactive species may be on or near surface negative electrode 110,the electroactive species may be in the interior of at least a portionof negative electrode 110, or a combination of the both. In someembodiments, some or all of the electroactive species is not a part ofan electrode. Instead, in some embodiments, the electroactive species ispresent in a conductive medium, such as an electrolyte (e.g., a liquidelectrolyte solution). In some such embodiments, the electroactivespecies can freely diffuse in a conductive medium (e.g., dissolved inconductive liquid such as a liquid electrolyte solution).

As used herein, a negative electrode of an electrochemical cell refersto an electrode into which electrons are injected during a chargingprocess. For example, referring to FIG. 2 , when electrochemical cell100 is charged (e.g., via the application of a potential by an externalpower source), electrons pass through an external circuit (not shown)and into negative electrode 110. As such, in some cases, species inelectronic communication with the negative electrode can be reduced to areduced state (a state having an increased number of electrons) during acharging process of the electrochemical cell.

The electroactive species may, in at least one conductive medium, havean oxidized state (having fewer electrons than the reduced state) and atleast one reduced state (having more electrons than the oxidized state).As a non-limiting example, if the electroactive species is anoptionally-substituted quinone, the neutral quinone would be consideredthe oxidized state, the semiquinone (product of the addition of oneelectron to the neutral quinone) would be considered one reduced state,and the quinone dianion (the product of the addition of one electron toneutral quinone) would be considered another reduced state.

In some embodiments, the electroactive species has, in at least oneconductive medium, at least one reduced state in which the species iscapable of bonding with the first Lewis acid gas (e.g. SO₂). A speciesbeing capable of bonding with a first Lewis acid gas generally refers toan ability for the species to undergo a bonding reaction with the firstLewis acid gas to a significant enough extent and at a rate significantenough for a useful gas capture and/or separation process to occur. Forexample, a species capable of bonding with a first Lewis acid gas mayhaving a binding constant with the first Lewis acid gas of greater thanor equal to 10¹ M⁻¹, greater than or equal to 10² M⁻¹, and/or up to 10³M⁻¹, or higher at room temperature (23° C.). A species capable ofbonding with a first Lewis acid gas may be able to bond with the firstLewis acid gas on a timescale of on the order of minutes, on the orderof seconds, on the order of milliseconds, or as low as on the order ofmicroseconds or less. A species may be capable of bonding with a Lewisacid gas at at least one temperature (e.g., at least one temperaturegreater than or equal to 223 K and less than or equal to 573K, such asat 298 K). In some embodiments, the species is capable of bonding with aLewis acid gas at a first temperature but bonding with the Lewis acidgas at a second temperature is thermodynamically and/or kineticallyunfavorable. Such a temperature dependence may be based on a temperaturedependence of a change in Gibbs free energy between the species (e.g.,reduced quinone) and the Lewis acid gas (e.g., carbon dioxide). With theinsight and guidance of this disclosure, one of ordinary skill in theart would be able to select an appropriate temperature for promotingbonding between the species in its at least one reduced state and theLewis acid gas (e.g., the first Lewis acid gas).

In some embodiments, the electroactive species has, in at least oneconductive medium, an oxidized state in which it is capable of releasingbonded first Lewis acid gas. The electroactive species may be chosensuch that in at least one reduced state it has a strong affinity for theLewis acid gas for the particular application for which it is intended.For example, in some embodiments, where SO₂ is the first Lewis acid gas,the chosen electroactive species may have a binding constant with SO₂ of10¹ to 10³ M⁻¹. In some embodiments, the chosen electroactive speciesmay have a binding constant with a different first Lewis acid gas of 10¹to 10³ M⁻¹. It has been observed that some, but not all quinones can beused as suitable electroactive species. In some embodiments, in thepresence of SO₂, an optionally-substituted quinone may be reduced to itssemiquinone or dianion (e.g., in a single step or multiple steps), whichthen binds to SO₂ forming a complex. Other electroactive species thatcan form a covalent bond with the first Lewis acid gas (SO₂), uponreduction may also be used.

In some embodiments, the electroactive species has, in at least oneconductive medium, at least one reduced state in which the species iscapable of bonding with the first Lewis acid gas, but for which there isat least one temperature (e.g., 298 K) at which it is thermodynamicallyunfavorable for the species to react with a second Lewis acid gas. Insome embodiments, the electroactive species has at least one reducedstate in which the species is capable of bonding with the first Lewisacid gas, but for which it is thermodynamically unfavorable for thespecies to react with the second Lewis acid gas at at least onetemperature in a range of greater than or equal to 223 K, greater thanor equal to 248 K, greater than or equal to 273 K, greater than or equalto 298 K, and/or up to 323 K, up to 348 K, up to 373 K, up to 398 K, upto 423 K, up to 448 K, up to 473 K, up to 498 K, up to 523 K, up to 548K, up to 573 K or higher. In some embodiments, the electroactive specieshas at least one reduced state in which the species is capable ofbonding with the first Lewis acid gas, but for it is thermodynamicallyunfavorable for the species to react with the second Lewis acid gas at atemperature of 298 K. It should be understood that a reaction beingthermodynamically unfavorable at a given temperature, as used herein,refers to the reaction having a positive change in Gibbs free energy(ΔG_(rxn)) at that temperature. For example, the reaction between thespecies in the at least one reduced state and the second Lewis acid gas(e.g., CO₂) may have a change in Gibbs free energy (ΔG_(rxn)) of greaterthan 0 kcal/mol, greater than or equal to +0.1 kcal/mol, greater than orequal to +0.5 kcal/mol, greater than or equal to +1 kcal/mol, greaterthan or equal to +2 kcal/mol, greater than or equal to +3 kcal/mol,greater than or equal to +5 kcal/mol, and/or up to +8 kcal/mol, up to+10 kcal/mol, up to +20 kcal/mol, or more at at least one temperature ina range of greater than or equal to 223 K, greater than or equal to 248K, greater than or equal to 273 K, greater than or equal to 298 K,and/or up to 323 K, up to 348 K, up to 373 K, up to 473 K, up to 573 K,or higher. In some embodiments, the reaction between the species in theat least one reduced state and the second Lewis acid gas (e.g., CO₂) hasa change in Gibbs free energy (ΔG_(rxn)) of greater than 0 kcal/mol,greater than or equal to +0.1 kcal/mol, greater than or equal to +0.5kcal/mol, greater than or equal to +1 kcal/mol, greater than or equal to+2 kcal/mol, greater than or equal to +3 kcal/mol, greater than or equalto +5 kcal/mol, and/or up to +8 kcal/mol, up to +10 kcal/mol, up to +20kcal/mol, or more at a temperature of 298 K.

In certain cases, the electroactive species has, in at least oneconductive medium, at least one reduced state in which the species iscapable of bonding with the first Lewis acid gas, but for which there isat least one temperature (e.g., 298 K) at which it is kineticallyunfavorable for the species to bond with the second Lewis acid gasbecause a rate for the reaction is too low for a reaction to occur on atimescale commensurate with the characteristic timescale for theprocess/process step (e.g., gas capture), such as microseconds,milliseconds, seconds, or minutes. It has been realized that such akinetic selectivity can be achieved in a variety of ways, includingfunctionalizing electroactive species with certain substituents. Forexample, the electroactive species may be functionalized with bulkysubstituents (e.g., tert-butyl moieties) such that steric hindranceimpedes reaction of the second Lewis acid gas with the species to agreater extent than it impedes reaction of the first Lewis acid gas withthe species.

In some embodiments in which the electroactive species has at least onereduced state in which the species is capable of bonding with the firstLewis acid gas, but for which there is at least one temperature (e.g.,298 K) at which it is kinetically unfavorable for the species to bondwith the second Lewis acid gas, a ratio of the rate constant for thereaction of the first Lewis acid gas with the species to the rateconstant for the reaction of the second Lewis acid gas is greater thanor equal to 2, greater than or equal to 5, greater than or equal to 10,greater than or equal to 50, greater than or equal to 100, and/or up to500, up to 1000, or greater. In some embodiments in which theelectroactive species has at least one reduced state in which thespecies is capable of bonding with the first Lewis acid gas, but forwhich there is at least one temperature (e.g., 298 K) at which it iskinetically unfavorable for the species to bond with the second Lewisacid gas, a ratio of the timescale of the reaction of the first Lewisacid gas with the species to the timescale of the second Lewis acid gasis greater than or equal to 2, greater than or equal to 5, greater thanor equal to 10, greater than or equal to 50, greater than or equal to100, and/or up to 500, up to 1000, or greater. A ratio of the timescalesof reactions of the first Lewis acid gas with the species and the secondLewis acid gas with the species can be determined by measuring the timeto 50% completion for each reaction, each reaction taking place underotherwise essentially identical conditions (same initial concentrationsof gas, same temperature same concentration, same reaction medium (e.g.,solvent and supporting electrolyte if present), same mixing rate, sameconcentration and/or accessible surface area of electroactive species,etc.). One of ordinary skill in the art, with the benefit of thisdisclosure, would be able to determine whether a reaction between aspecies and a Lewis acid gas is thermodynamically and/or kineticallyfavorable or unfavorable using, for example, cyclic voltammetry in theconductive medium (with the conductive medium saturated with the Lewisacid gas).

It is believed that the pK_(a) of the electroactive species in itsreduced states may contribute at least in part to control of theselectivity of the species with respect to the first Lewis acid gas andthe second Lewis acid gas. In some embodiments, in the at least onereduced state the electroactive species comprises a moiety having apK_(a) that is greater than or equal to the pK_(a) of the first Lewisacid gas and less than the pK_(a) of the second Lewis acid gas. As anon-limiting example, in some embodiments, in the at least one reducedstate the electroactive species comprises a moiety (e.g., carbonylgroup) having a pK_(a) that is greater than or equal to SO₂ and lessthan the pK_(a) of CO₂ in at least one conductive medium, or in theconductive medium of the process being performed. By judiciouslychoosing the pKa of the electroactive species in its reduced states(e.g., by derivatization with functional groups), selective reactivitywith the first Lewis acid gas relative to the second Lewis acid gas canbe achieved. One of ordinary skill in the art would be able to determinethe pK_(a) of an electroactive species (e.g., in a reduced state) bychemically or electrochemically preparing the reduced state andperforming an acid base titration (e.g., colorimetrically, via cyclicvoltammetry, etc.), or any other suitable technique known in the art.The relative pK_(a) values of species under given conditions(temperature, solvent, supporting electrolyte) can be determined usingelectrochemical techniques such as cyclic voltammetry or open-circuitpotential techniques to determine the reduction potentials of thespecies, the species with the more positive reduction potential havingthe lower pK_(a). The pK_(a) may depend on the temperature at which itis measured. In some embodiments, the pK_(a) is measured at any of thetemperatures mentioned above, such as at 298 K.

In some instances, the electroactive species in its at least one reducedstate may be capable of reacting with both the first Lewis acid gas andthe second Lewis acid gas at a first temperature, but at a second,different temperature, the electroactive species in its at least onereduced state is capable of bonding with a first Lewis acid gas, but areaction with the second Lewis acid is thermodynamically and/orkinetically unfavorable. In some such instances, removing an amount ofthe first Lewis acid gas from a fluid mixture comprising the first Lewisacid gas and the second Lewis acid gas may comprise bonding both thefirst Lewis acid gas to one or more electroactive species in a reducedstate and bonding the second Lewis acid gas to the one or moreelectroactive species in the reduced state at a first temperature toform first Lewis acid gas-electroactive species complexes and secondLewis acid gas-electroactive species complexes, respectively.Subsequently, the first Lewis acid gas-electroactive species complexesand second Lewis acid gas-electroactive species complexes may be exposedcondition at a second, different temperature that results in release ofan amount (e.g., at least 10%, at least 25%, at least 50%, at least 75%,at least 90%, at least 95%, at least 98%, at least 99%, or all by molepercent or volume percent) of the second Lewis acid gas from thecomplexes via reversal of the bonding reaction between the second Lewisacid gas and the electroactive species, while releasing essentially noneor relatively little of the first Lewis acid gas (e.g., less than orequal to 10%, less than or equal to 5%, less than or equal to 2%, lessthan or equal to 1%, less than or equal to 0.5%, less than or equal to0.1%, and/or as little as 0.05%, as little as 0.01%, or less by molepercent or volume percent). Changing from the first temperature to thesecond temperature may cause at least partial release by shiftingequilibrium constants for the respective bonding reactions towardrelease of the gas. For example, the complex-forming bonding reactionsmay each have a negative Gibbs free energy change at the firsttemperature, but at the second temperature the reaction involvingbonding of the first Lewis acid gas may remain negative while thereaction involving bonding of the second Lewis acid may become positive(and therefore thermodynamically unfavorable). Judicious choice ofelectroactive species may be employed to achieve such an effect based ona variety of considerations. For example, an electroactive species maybe chosen based on knowledge or measurements of changes in enthalpy andchanges in entropy for the respective complex-forming reactions with theelectroactive species.

One non-limiting way in which the first Lewis acid gas may be removedwhile removing little to essentially none of the second Lewis acid gas(e.g., carbon dioxide) from the fluid mixture is by applying a certainpotential across the electrochemical cell during at least a portion ofthe operation. For example, it is has been discovered in the context ofthe present disclosure that it is possible to apply a potential acrossthe electrochemical cell (e.g., first potential) that is sufficient toreduce the electroactive species to at least one reduced state in whichit is capable of bonding to the first Lewis acid gas, but the potentialis insufficient to reach a state in which the species (or the electrodeitself) is capable of reacting (e.g., binding) with the second Lewisacid gas. Judicious choice of electroactive species may allow for such apotential to be applied, whereas certain conventional electroactivespecies may not allow for such a potential to be applied. The potentialapplied across the electrochemical cell may be such that the electrodepotential at the negative electrode is positive (e.g., by greater thanor equal to 10 mV, greater than or equal to 50 mV, greater than or equalto 100 mV, greater than or equal to 200 mV, greater than or equal to 5mV, and/or up to 1 V or more) relative to the standard reductionpotential for the formation of a reduced state of the species capable ofbonding to the second Lewis acid gas.

While certain embodiments described above relate to selective removal ofa first Lewis acid gas from a mixture comprising a first Lewis acid gasand a second Lewis acid gas via selectively reacting the first Lewisacid gas with an electroactive species to a greater extent than that ofthe second Lewis acid gas, other methods of selective removal of thefirst Lewis acid gas are contemplated as well. As one example, someembodiments are related to methods involving selective Lewis acid gasremoval by bonding a first Lewis acid gas (e.g., sulfur dioxide) and asecond Lewis acid gas (e.g., carbon dioxide) to one or more reducedelectroactive species and, subsequently, selectively releasing thesecond Lewis acid gas (e.g., via oxidation of a second Lewis acidgas-electroactive species complex) from the complexes while releasingrelatively little or none of the first Lewis acid gas from the complexes

In some embodiments, a fluid mixture comprising a first Lewis acid gasand a second Lewis acid gas is exposed to one or more electroactivespecies. The electroactive species (e.g., optionally-substitutedquinones) may be in a reduced state (e.g., an optionally-substitutedsemiquinone, an optionally substituted quinone dianion, or combinationsthereof). For example, referring to FIG. 3 , fluid mixture 101comprising first Lewis acid gas 102 and second Lewis acid gas 104 may beexposed to reduced electroactive species R. The electroactive speciesmay initially be in an oxidized state (e.g., an optionally-substitutedquinone) and then converted to a reduced state (e.g., anoptionally-substituted semiquinone or quinone dianion). Such a reductionprocess to prepare the electroactive species in the reduced state mayoccur prior to the step of exposing the fluid mixture comprising a firstLewis acid gas and a second Lewis acid gas to the electroactive species,and/or during the exposure. The reduction may occur, for example, viaelectron transfer upon applying an electrical potential differenceacross an electrochemical cell comprising a negative electrode inelectronic communication with the electroactive species. Theelectroactive species in the reduced state may be part of an electrode(e.g., immobilized on a negative electrode), freely diffusing in aliquid solution (e.g., the fluid mixture), or a combination thereof.

Exposure to the one or more electroactive species in their reduced statemay, for example, comprise flowing the fluid mixture to or past theelectroactive species (e.g., in proximity to the electroactive species)and/or mixing the Lewis acid gases with the electroactive species insolution (e.g., via mixing separate solutions or bubbling a solutioncomprising the electroactive species with a gas mixture comprising theLewis acid gases).

In some embodiments, an amount of the first Lewis acid gas is bonded toa first portion the electroactive species in the reduced state to formfirst Lewis acid gas-electroactive species complexes. Further, in someembodiments, an amount of the second Lewis acid gas is bonded to asecond portion of the electroactive species in the reduced state to formsecond Lewis acid gas-electroactive species complexes. The bonding ofthe first Lewis acid gas to the first portion of the electroactivespecies and the bonding of the second Lewis acid gas to the secondportion of the electroactive species may occur simultaneously orsequentially. For example, in some embodiments, the second Lewis acidgas may be bonded to the reduced electroactive species (e.g., a subsetor all of the electroactive species) and then, after a period of time,the first Lewis acid gas may be bonded to the reduced electroactivespecies (e.g., a subset or all of the electroactive species). In otherembodiments, the first Lewis acid gas and the second Lewis acid gas areeach bonded to the reduced electroactive species during a same period oftime. It should be understood that Lewis acid gas-electroactive speciescomplexes can be formed by any of a variety of forces, such a covalentbonds, ionic bonds, hydrogen bonds, or specific noncovalent affinityinteractions. Referring again to FIG. 3 , first Lewis acid gas molecules102 (e.g., sulfur dioxide) may bond to reduced electroactive species Rto form first Lewis acid gas-electroactive species complexes 107, andsecond Lewis acid gas molecules 104 (e.g., carbon dioxide) may bond toreduced electroactive species R to form second Lewis acidgas-electroactive species complexes 109, according to certainembodiments. The first and second Lewis acid gases, when complexed, maybe at least temporarily immobilized with respect to a structure of adevice (e.g., an electrode) or with respect to a solution in which theelectroactive species is present (e.g., dissolved). The first portion ofthe one or more electroactive species (to which the first Lewis acid gasis bonded) may, for example, be a first plurality of electroactivespecies molecules or polymer moieties, and the second portion of theelectroactive species (to which the second Lewis acid gas is bonded) maybe a second plurality of electroactive species molecules or polymermoieties. The first portion of the one or more electroactive species andthe second portion of the one or more electroactive species may be thesame type of species (e.g., same type of optionally-substituted quinonemolecules or polymer residues)), or the first portion and second portionmay include different types of species (e.g., quinones having differingsubstituents).

In some embodiments, at least some of the second Lewis acidgas-electroactive species complexes are oxidized such that an amount(e.g., at least 10%, at least 25%, at least 50%, at least 75%, at least90%, at least 95%, at least 98%, at least 99%, or all by mole percent orby volume percent) of the second Lewis acid gas is released from thecomplexes. For example, referring again to FIG. 3 , second Lewis acidgas-electroactive species complexes 109 may be oxidized during step 10to form the electroactive species in their oxidized state Ox, therebyreleasing second Lewis acid gas molecules 104 from the second Lewis acidgas-electroactive species complexes 109. The released second Lewis acidgas molecules (e.g., carbon dioxide) may then, in some instances, beseparated from the fluid mixture comprising the first Lewis acidgas-electroactive species complexes (e.g., sulfur dioxide-electroactivespecies complexes). Such a separation can be accomplished in some casesin which the complexes are immobilized by flowing the fluid mixture pastthe immobilized complexes (e.g., by flowing a gas stream or fluid streamusing positive and/or negative pressure sources). In some cases in whichthe complexes are at least partially dissolved in solution, the secondLewis acid gas may be separated by out-gassing the second Lewis acid gasor via evaporation (e.g., via exposure to reduced pressure conditionssuch as exposure to vacuum). The oxidation of the second Lewis acidgas-electroactive species complexes may be performed using any of avariety of techniques such as electrochemically or chemically. Forexample, the oxidation step may comprise exposing the second Lewis acidgas-electroactive species complexes to an electrochemical cell whileapplying an electrical potential difference across the electrochemicalcell. The second Lewis acid gas-electroactive species complexes may beexposed, for example to a negative electrode of the electrochemical celleither as free complexes in solution (under diffusion or forced fluidflow), or the electroactive species may be immobilized with respect tothe negative electrode (e.g., via adsorption, functionalization, orinclusion in a redox-active polymer). The oxidation may, alternativelyor additionally, involve the exposure to a suitable chemical oxidizingagent dissolved in solution or immobilized/deposited on a surface.

In some embodiments, while at least some of the second Lewis acidgas-electroactive species complexes are oxidized (and an amount of thesecond Lewis acid gas is released), essentially none (e.g., a negligibleamount with respect to the application being employed) of the firstLewis acid gas is released from the first Lewis acid gas-electroactivespecies complexes. In some embodiments, while at least some of thesecond Lewis acid gas-electroactive species complexes are oxidized (andan amount of the second Lewis acid gas is released), an amount of thefirst Lewis acid gas is released that is less than or equal to 70%, lessthan or equal to 50%, less than or equal to 25%, less than or equal to10%, less than or equal to 5%, less than or equal to 2%, less than orequal to 1%, less than or equal to 0.5%, less than or equal to 0.1%,less than or equal to 0.05%, and/or as low as 0.01%, as low as 0.001%,or less of the first Lewis acid gas-electroactive species complexes bymole percent. Essentially none or relatively little first Lewis acid gasmay be released during oxidation of the second Lewis acidgas-electroactive species complexes for any of a variety reasons. Forexample, the conditions under which oxidation of the second Lewis acidgas-electroactive species complex occurs may not lead to oxidation ofthe first Lewis acid gas-electroactive species complex because theoxidizing power of an oxidizing agent (e.g., a chemical oxidant or anelectrode at a given electrical potential) may be sufficient to oxidizethe second Lewis acid gas-electroactive species complex (e.g.,thermodynamically or kinetically), but insufficient to oxidize the firstLewis acid gas-electroactive species complex. Such an occurrence mayhappen when the different complexes have different oxidation potentialsunder the given conditions, which may be attributable to differingacidities of the first Lewis acid gas and the second Lewis acid gas.Another example is where the conditions under which oxidation of thesecond Lewis acid gas-electroactive species complexes occurs alsoresults in oxidation of at least some of the first Lewis acidgas-electroactive species complexes, but where an affinity between thefirst Lewis acid gas and the electroactive species in its oxidized stateis strong enough that a complex is maintained and the first Lewis acidgas is not released. Judicious choice of oxidizing agent/electricalpotential and/or electroactive species (e.g., based on measuredreduction potentials and/or pK_(a) values of the electroactive speciesand Lewis acid gases) may be used to employ any of the techniquesdescribed above.

In some embodiments, the step of oxidizing the second Lewis acidgas-electroactive species complexes is performed multiple times. Forexample, the gas released during the oxidizing step (e.g., releasedsecond Lewis acid gas and a relatively small amount of first Lewis acidgas) may be separated from the fluid mixture and exposed to a second setof reduced electroactive species (e.g., at a second electrochemicalcell) to form new second Lewis acid gas-electroactive species complexesand first Lewis acid gas-electroactive species, wherein a ratio of thefirst Lewis acid gas to second Lewis acid gas is higher than during theinitial exposure step of the process. Then, the new second Lewis acidgas-electroactive species may be oxidized to release second Lewis acidgas from the complexes, while releasing essentially none or a relativelysmall amount of the first Lewis acid gas from the complexes. Thisprocess may be repeated, two, three, four, or more times, with eachprocess progressively enriching a fluid mixture with the second Lewisacid gas while depleting it of the first Lewis acid gas. Such asequential process could be performed using, for example, adistillation-type apparatus having multiple trays.

In some embodiments, the first Lewis acid gas-electroactive speciescomplexes are oxidized such that an amount (e.g., at least 10%, at least25%, at least 50%, at least 75%, at least 90%, at least 95%, at least98%, at least 99%, or all by mole percent or by volume percent) of thefirst Lewis acid gas is released. Such a release of the first Lewis acidgas (e.g., sulfur dioxide) may occur after separation from the secondLewis acid gas (e.g., carbon dioxide). In some embodiments, oxidation ofthe first Lewis acid gas-electroactive species complexes occurs afteroxidation of the second Lewis acid gas-electroactive species complexes.As an example, in some embodiments, the oxidation of the second Lewisacid gas-electroactive species complexes is a first oxidizing stepperformed during a first period of time, and a second oxidizing stepcomprising oxidizing at least some of the first Lewis acidgas-electroactive species complexes is performed during a second (e.g.,later) period of time such that an amount of the first Lewis acid gas isreleased. Referring again to FIG. 3 , first Lewis acid gas-electroactivespecies complex 107 may be oxidized during step 20 to form theelectroactive species in their oxidized state Ox, thereby releasingfirst Lewis acid gas molecules 102.

The second oxidizing step may, as in the case of the first oxidizingstep, be performed electrochemically or chemically. For example, thesecond oxidizing step may comprise exposing the second Lewis acidgas-electroactive species complexes to an electrochemical cell whileapplying an electrical potential difference across the electrochemicalcell. In some embodiments, the electrochemical cell used for the secondoxidation step is the same electrochemical cell as the first oxidationstep. For example, the second Lewis acid gas-electroactive speciescomplexes may be oxidized at a negative electrode of an electrochemicalcell during a first period of time, and then the first Lewis acidgas-electroactive species complexes may be oxidized at the same negativeelectrode during a second period of time. The different oxidations mayoccur at different times by employing different electrical potentials atthe different times. For example, a first electrical potentialdifference may be applied during the first oxidizing step, at amagnitude sufficient to oxidize the second Lewis acid gas-electroactivespecies complexes but insufficient (e.g., thermodynamically) to oxidizethe first Lewis acid gas-electroactive species complexes. Then, duringthe second oxidizing step, a second electrical potential difference maybe applied at a magnitude sufficient to oxidize the first Lewis acidgas-electroactive species complexes.

Alternatively, the first and second oxidations may be performed atdifferent electrochemical cells (e.g., of a gas separation system). Forexample, the first oxidizing step may comprise exposing the second Lewisacid gas-electroactive species complexes to a first electrochemical cellwhile applying an electrical potential difference across the firstelectrochemical cell, and the second oxidizing step may compriseexposing the first Lewis acid gas-electroactive species complexes to asecond (different) electrochemical cell while applying an electricalpotential difference across the second electrochemical cell. The secondelectrical potential difference may be different than the firstelectrical potential difference (e.g., resulting in a more positiveelectrical potential at a negative electrode). Such a process may beperformed using, for example, a redox flow apparatus. FIG. 4 shows aschematic illustration of exemplary flow apparatus 400 comprising firstelectrochemical cell 100 and second electrochemical cell 200. Firstelectrochemical cell 100 and second electrochemical cell 200 may eachcomprise an anode 110. Anodes 110 may each be in fluidic communicationwith conduit 402 configured to flow fluid mixtures (e.g., gaseous fluidmixtures or liquid solutions). That is, fluid in the conduit may becapable of contacting at least one surface of anodes of the firstelectrochemical cell and the second electrochemical cell. In theembodiment shown in FIG. 4 , fluid apparatus 400 may be configured toreceive fluid mixture 405 (e.g., from a fluid mixture source) via aninlet, and anode 110 of first electrochemical cell 100 may be arrangedwith the conduit such that flow of fluid mixture 405 may expose fluidmixture 405 to anode 110 of first electrochemical cell 100. At firstelectrochemical cell 100, oxidizing step 10 shown in FIG. 3 may beperformed to oxidize second Lewis acid gas-electroactive speciescomplexes such that an amount of second Lewis acid gas is released. Anintermediate outlet 403 may be positioned and configured to receivesecond Lewis acid gas 407 separated from fluid mixture 405 (e.g., viaconnection to a vacuum source). Flow apparatus 406 may be configured totransport the resulting fluid mixture 406 comprising first Lewis acidgas-electroactive species but at least partially (or completely)depleted of second Lewis acid gas molecules to second electrochemicalcell 200. Anode 110 of second electrochemical cell 200 may be arrangedwith conduit 402 such that flow of fluid mixture 406 may expose fluidmixture 406 to anode 110 of second electrochemical cell 200. At secondelectrochemical cell 200, oxidizing step 20 shown in FIG. 3 may beperformed to oxidize first Lewis acid gas-electroactive speciescomplexes such that an amount of first Lewis acid gas is released. Flowapparatus 400 may further be configured to expel released first Lewisacid as (e.g., as gas 408 from an outlet).

As one non-limiting example of a process described above, sulfur dioxideand carbon dioxide may each be exposed to dissolved para-naphthoquinonedianion (p-NQ²⁻) in an organic liquid (e.g., via bubbling of the gasesinto the liquid). The para-naphthoquinone dianion may be prepared viaelectrochemical or chemical reduction. The exposure may result information of p-NQ(SO₂)₂ and p-NQ(CO₂)₂ complexes in solution. Thesolution may then be exposed to a negative electrode of anelectrochemical cell during application of an oxidizing potentialsufficient to oxidize the p-NQ(CO₂)₂ to form neutral species p-NQ andCO₂, but insufficient to oxidize the p-NQ(SO₂)₂ complexes in solution.The released CO₂ may be removed from the solution (e.g., as part of afluid mixture such as a gas mixture for further downstream processingsuch as carbon capture). The remaining solution comprising p-NQ(SO₂)₂may then be exposed to a negative electrode (either the same electrodeor an electrode of a second electrochemical cell) during application ofa more positive oxidizing potential sufficient to oxidize p-NQ(SO₂)₂ toform p-NQ and SO₂, now separated from the CO₂.

The electroactive species described herein may be of any suitable form,provided that it satisfies at least one of the criteria required herein.In some embodiments, the electroactive species is or comprises amolecular species. For example, the electroactive species may be orcomprise an organic molecule. The electroactive species may comprise oneor more functional groups capable of binding to a first Lewis acid gasin a fluid mixture (e.g., when the electroactive species is in a reducedstate). The functional groups may include, for example, a carbonylgroup. In some embodiments, the electroactive species is part of apolymer, such as a redox-active polymer. The electroactive species maybe part of a polymeric material immobilized on the negative electrode.For example, referring to FIG. 2 , the electroactive species may be partof a polymeric material immobilized on negative electrode 110 ofelectrochemical cell 100. As mentioned above, however, the electroactivespecies may be present in a conductive medium (e.g., a conductiveliquid).

In some embodiments, the electroactive species is or comprises anorganic species. The species may be optionally-substituted (i.e., thespecies may comprise functional groups and/or other moieties or linkagesbonded to the main structure of the species) In some embodiments, theorganic species comprises one or more species chosen fromoptionally-substituted quinone, optionally-substituted thiolate, anoptionally-substituted bipyridine, an optionally-substituted phenazine,and an optionally-substituted phenothiazine.

In certain cases, the electroactive species is or comprises aredox-active polymer comprising an optionally-substituted organicspecies. The choice of substituent (e.g., functional groups) on theoptionally-substituted species may depend on any of a variety offactors, including but not limited to its effect on the pK_(a) and/orthe standard reduction potential of the optionally-substituted species.One of ordinary skill, with the benefit of this disclosure, wouldunderstand how to determine which substituents or combinations ofsubstituents on the optionally-substituted species (e.g., quinone) aresuitable for the electroactive species based on, for example syntheticfeasibility, and resulting pK_(a) and/or standard reduction potential.

As a non-limiting example, it has been discovered that substitution ofcertain quinones with electron-withdrawing groups can modulate theelectron density of certain redox states of the quinone which may affectthe species' selectivity for Lewis acid gases (e.g., by modulating thepK_(a) of the reduced state). As one non-limiting example, it has beenobserved unexpectedly that functionalizing 1,4-naphthoquinone withelectron withdrawing groups (e.g., nitriles to form2,3-dicyano-1,4-napthoquinone) can impart selectivity for binding of SO₂over binding CO₂ upon reduction. With the benefit of this insight, oneor ordinary skill could screen potential electroactive species for adesired selectivity for Lewis acid gases by performing cyclicvoltammetry and/or thermogravimetric analysis of the electroactivespecies in a desired conductive liquid at a desired temperature in thepresence of the each Lewis acid gas, and determining relativereactivities.

In some embodiments, the optionally-substituted quinone is or comprisesan optionally-substituted naphthoquinone. In certain cases, theoptionally-substituted quinone is or comprises an optionally-substitutedanthraquinone. In some embodiments, the optionally substituted quinoneis or comprises an optionally-substituted quinoline. In someembodiments, the optionally-substituted quinone is or comprises anoptionally-substituted thiochromene-dione. In some embodiments, theoptionally-substituted quinone is one of benzo[g]quinoline-5,10-dione,benzo[g]isoquinoline-5,10-dione, benzo[g]quinoxaline-5,10-dione,quinoline-5,8-dione, or 1-lamba⁴-thiochromene-5,8-dione. In someembodiments, the optionally-substituted quinone is or comprises anoptionally-substituted phenanthrenequinone (also referred to as anoptionally-substituted phenanthrenedione). The substituents (e.g.,functional groups) may be any of those listed above or below.

As mentioned above, the electroactive species may be part of aredox-active polymer. In some cases, any of the optionally-substitutedspecies (e.g., organic species) described herein may be part of theredox-active polymer. In some such cases, at least a portion of theredox-active polymer comprises a backbone chain and one or more of theoptionally-substituted species covalently bonded to the backbone chain.A backbone chain generally refers to the longest series of covalentlybonded atoms that together create a continuous chain of the polymermolecule. In certain other cases, the optionally-substituted speciesdescribed herein may be part of the backbone chain of the redox-activepolymer.

The electroactive species may comprise cross-linked polymeric materials.For example, in some embodiments, the electroactive species comprises oris incorporated into hydrogels, ionogels, organogels, or combinationsthereof. Such cross-linked polymeric materials are generally known inthe art, and may in some instances comprise electroactive speciesdescribed herein as part of the three-dimensional structure (e.g., viacovalent bonds). However, in some embodiments, electroactive species areincorporated into the cross-linked polymeric materials via adsorption(e.g., physisorption and/or chemisorption). In some embodiments, theelectroactive species comprises an extended network. For example, theelectroactive species may comprise a metal organic framework (MOF) or acovalent organic framework (COF). In some embodiments, the electroactivespecies comprises functionalized carbonaceous materials. For example,the electroactive species may comprise functionalized graphene,functionalized carbon nanotubes, functionalized carbon nanoribbons,edge-functionalized graphite, or combinations thereof.

Exemplary functional groups with which the optionally-substitutedquinone may be functionalized include, but are not limited to, halo(e.g., chloro, bromo, iodo), hydroxyl, carboxylate/carboxylic acid,sulfonate/sulfonic acid, alkylsulfonate/alkylsulfonic acid,phosphonate/phosphonic acid, alkylphosphonate/alkylphosphonic acid, acyl(e.g., acetyl, ethyl ester, etc.), amino, amido, quaternary ammonium(e.g., tetraalkylamino), branched or unbranched alkyl (e.g., C1-C18alkyl), heteroalkyl, alkoxy, glycoxy, polyalkyleneglycoxy (e.g.,polyethyleneglycoxy), imino, polyimino, branched or unbranched alkenyl,branched or unbranched alkynyl, aryl, heteroaryl, heterocyclyl, nitro,nitrile, thiyl, and/or carbonyl groups, any of which isoptionally-substituted. The above-mentioned functional groups may alsobe employed in any of the other types of electroactive species describedherein (e.g., optionally-substituted thiolate, an optionally-substitutedbipyridine, an optionally-substituted phenazine, and anoptionally-substituted phenothiazine, functionalized hydrogels,functionalized carbonaceous materials such as functionalized graphene,functionalized carbon nanotubes, edge-functionalized graphite, etc.). Aswould be understood by a person of ordinary skill in the art, aheteroaryl substitution of an aromatic species such as a quinone may bea ring fused with the aromatic species. For example, a quinonefunctionalized with a heteroaryl group can be a quinoline-dione (e.g., abenzoquinoline-dione). Heteroatoms in rings that are part ofelectroactive species, may, in some instances, affect the pK_(a) of areduced form of the electroactive species and/or its standard reductionpotential. For example, a quinoline-dione may have a more positivestandard reduction potential than a naphthoquinone, and aquinoxaline-dione may have a more positive standard reduction potentialthan the quinoline-dione.

In certain aspects, electrochemical apparatuses are generally described.FIG. 2 depicts electrochemical apparatus 105 as one such example,according to certain embodiments. The electrochemical apparatus may, insome instances, be configured to perform the methods described herein.

In some embodiments, the electrochemical apparatus comprises a chambercomprising a negative electrode. For example, in some embodiments,electrochemical apparatus 105 comprises chamber 103 and electrochemicalcell 100, which comprises negative electrode 110. The chamber may beconstructed for receiving a fluid mixture. In some instances, thechamber of the electrochemical apparatus is configured such that a fluidmixture can enter the chamber and in some instances leave the chamber.For example, in some embodiments, the chamber comprises a fluid inletand a fluid outlet. Referring again to FIG. 2 , in some embodiments,electrochemical apparatus 105 comprises chamber 103 comprising fluidinlet 106 and fluid outlet 108. As such, one or more of the methodsdescribed herein may be performed by flowing fluid mixture 101 (e.g.,comprising a first Lewis acid gas and a second Lewis acid gas) intochamber 103 via fluid inlet 106, thereby exposing at least a portion ofthe fluid mixture to the electrochemical cell (e.g., including negativeelectrode 110). The electrochemical cell may be equipped with externalcircuitry and a power source (e.g., coupled to a potentiostat) to allowfor application of the potential difference. The electrochemicalapparatus may be configured such that at least a portion of the fluidmixture can be transported out of the chamber via a fluid outlet (e.g.,fluid outlet 108 in FIG. 2 ). In some embodiments, the fluid inlet isfluidically connected to a fluid mixture source (e.g., a source of amixture comprising a first Lewis acid gas and a second Lewis acid gas).In some embodiments the fluid outlet is fluidically connected to adownstream apparatus for further processing (e.g., anotherelectrochemical apparatus for removing another Lewis acid gas). In someembodiments, the electrochemical apparatus comprises a plurality of thechambers (e.g., each comprising a negative electrode) fluidicallyconnected in series.

In some embodiments, the electrochemical apparatus comprises theelectroactive species in electronic communication with the negativeelectrode. For example, referring again to FIG. 2 , in some embodiments,the electroactive species (not pictured) is in electronic communicationwith negative electrode 110. Electronic communication in this contextgenerally refers to an ability to undergo electron transfer reactions,either via outer sphere (electron/hole transfer) or inner sphere (bondbreaking and/or bond making) mechanisms. In some embodiments in whichthe electroactive species is in electronic communication with thenegative electrode, the electroactive species is immobilized on thenegative electrode. For example, the electroactive species may be partof a redox-active polymer immobilized on to the electrode via, in someinstances, a composite layer (e.g., comprising a carbonaceous materialsuch as carbon nanotubes). In some embodiments in which theelectroactive species is in electronic communication with the negativeelectrode, the electroactive species is present in a conductive mediumin at least a portion of the electrochemical cell, and can undergoelectron transfer reactions with the electrode (directly or indirectly).For example, the electroactive species may be present (e.g., dissolvedor suspended) in a conductive liquid of the electrochemical cell and beable to diffuse close enough to the negative electrode such that anelectron transfer reaction can occur (e.g., to reduce the electroactivespecies into at least one reduced state) upon application of thepotential difference across the electrochemical cell.

As mentioned above, in some embodiments, the first electroactive speciesis immobilized on the negative electrode. Such embodiments may bedistinguished from those of other embodiments, in which theelectroactive species are free to be transported from one electrode toanother via, for example, advection. A species immobilized on anelectrode (e.g., the negative electrode) may be one that, under a givenset of conditions, is not capable of freely diffusing away from ordissociating from the electrode. The electroactive species can beimmobilized on an electrode in a variety of ways. For example, in somecases, an electroactive species can be immobilized on an electrode bybeing bound (e.g., via covalent bonds, ionic bonds, and/orintramolecular interaction such as electrostatic forces, van der Waalsforces, hydrogen bonding, etc.) to a surface of the electrode or aspecies or material attached to the electrode. In some embodiments, theelectroactive species can be immobilized on an electrode by beingadsorbed onto the electrode. In some cases, the electroactive speciescan be immobilized on an electrode by being polymerized onto theelectrode. In certain cases, the electroactive species can beimmobilized on an electrode by being included in a composition (e.g., acoating, a composite layer, etc.) that is applied or deposited onto theelectrode. In certain cases, the electroactive species (e.g., polymericor molecular electroactive material) infiltrates a microfiber or,nanofiber, or carbon nanotube mat, such that the electroactive materialis immobilized with respect to the mat. The mat may provide an enhancedas surface area enhancement for electrolyte and gas access, as well asexpanded network for electrical conductivity. In some embodiments, theelectroactive species is part of a gel composition associated with theelectrode (e.g., as a layer deposited on the electrode, as a compositioninfiltrating pores of the electrode, or as a composition at leastpartially encapsulating components of the electrode such as fibers ornanotubes of the electrode). Such a gel comprising the electroactivespecies (e.g., a hydrogel, ionogel, organogel, etc.) may be preparedprior to association with the electrode (e.g., applied as a coating toform a layer), or the gel may be prepared in the presence of theelectrode by contacting the electrode (e.g., via coating or submersion)with a gel precursor (e.g., a pre-polymer solution comprising theelectroactive species) and gel formation may then be initiated (e.g.,via cross-linking via introduction of a crosslinking agent, a radicalinitiator, heating, and/or irradiation with electromagnetic radiation(e.g., ultraviolet radiation)).

In some embodiments, the electrochemical of the electrochemicalapparatus further comprises a positive electrode. In some, but notnecessarily all embodiments, the electrochemical cell comprises aseparator between the negative electrode and the positive electrode. Forexample, referring to FIG. 2 , in some embodiments, electrochemical cell100 comprises optional separator 130 between negative electrode 110 andoptional positive electrode 120. As used herein, a positive electrode ofan electrochemical cell refers to an electrode from which electrons areremoved during a charging process. For example, referring again to FIG.2 , when electrochemical cell 100 is charged (e.g., via the applicationof a potential by an external power source), electrons pass frompositive electrode 120 and into an external circuit (not shown). Assuch, in some cases, species associated with the positive electrode, ifpresent, can be oxidized to an oxidized state (a state having adecreased number of electrons) during a charging process of theelectrochemical cell.

In some embodiments, the electroactive species in electroniccommunication with the negative electrode describe above is a firstelectroactive species and the positive electrode comprises a secondelectroactive species. The second electroactive species may be adifferent composition than the first electroactive species of thenegative electrode, though it some embodiments the second electroactivespecies is the same as the first electroactive species. In someembodiments the positive electrode comprises an electroactive layer(sometimes referred to as a complementary electroactive layer)comprising the second electroactive species. The complementaryelectroactive layer may be in the form of a composite, and as such, maybe a complementary electroactive composite layer. In operation, thissecond electroactive species may serve as a source of electrons for thereduction of the first electroactive species present in the negativeelectrode. Likewise, the second electroactive species may serve as asink for electrons during the oxidation of the first electroactivespecies. It is in this manner that the electroactive layer of thepositive electrode may be described as “complementary.” The secondelectroactive species may comprise, for example, a redox-active polymer.In some embodiments, the redox-active polymer is or comprises a polymercomprising ferrocene (e.g., as moieties bonded to the polymer backbone).In some embodiments, second electroactive species comprises ametallocene (e.g., ferrocene). In some such cases, the secondelectroactive species comprises a redox-active polymer comprising ametallocene. As one non-limiting embodiment, the redox-active polymercomprises polyvinyl ferrocene. As another example, the secondelectroactive species may comprise a polymer comprising a thiophene. Insome such cases, the second electroactive species comprisespoly(3-(4-fluorophenyl)thiophene). In some embodiments, the secondelectroactive species comprises phenothiazine. As another example, insome embodiments, the second electroactive species comprises(2,2,6,6-tetramethylpiperidin-1-yl)oxyl (referred to as “TEMPO”), orderivatives thereof (e.g., comprising optional substituents). In certaincases, the second electroactive species comprises a Faradaic redoxspecies having a standard reduction potential at least 0.5 volts (V), atleast 0.6 V, at least 0.8 V, and/or up to 1.0 V, up to 1.5 V, or morepositive than the first reduction potential of the first electroactivespecies.

In some embodiments, the second electroactive species comprises anintercalation compound. For example, the second electroactive maycomprise a metal ion intercalation compound. One exemplary class ofintercalation compounds includes metal oxides. The intercalationcompound may include intercalation compounds of alkali metal ions suchas lithium ions and/or sodium ions. In some embodiments, theintercalation compound comprises an alkali metal ion transition metaloxide (e.g., a lithium cobalt oxide, a lithium manganese oxide, alithium nickel oxide, and/or lithium oxides comprising cobalt,manganese, and/or nickel). In some embodiments, the intercalationcompound comprises an alkali metal transition metal polyoxyanion, suchas a lithium transition metal phosphate. One example of a suitablelithium transition metal phosphate for the positive electrode is lithiumiron phosphate (LiFePO₄). In some embodiments, during the charge mode,the oxidation of a second electroactive species in the form of an alkalimetal ion intercalation compound (e.g., LiFePO₄) provides a source ofelectrons for driving the reduction of the first electroactive species,while simultaneous releasing an alkali metal ion (e.g., a lithium ion)that can shuttle to through an electrolyte (e.g., on or within aseparator when present) toward the negative electrode to maintain chargebalance and complete an electrochemical circuit. Conversely, during adischarge mode, the reduction of a second electroactive species in theform of an alkali metal ion intercalation compound provides a sink forelectrons from the oxidation of the first electroactive species, whileat the same time an alkali metal ion (e.g., a lithium ion) can shuttlefrom the a region in proximity to the negative electrode, through anelectrolyte (e.g., on or within a separator when present), and towardthe positive electrode where it can be intercalated into theintercalation compound and maintain charge balance.

The complementary electroactive composite layer of the positiveelectrode may comprise an immobilized polymeric composite of anelectroactive species and of another material (e.g., a carbonaceousmaterial). Examples of the carbonaceous material include carbon nanotube(e.g., single-walled carbon nanotube, multi-walled-carbon nanotube),carbon black, KetjenBlack, carbon black Super P, or graphene. Othermaterials are also possible. In certain cases, the second electroactivespecies can be immobilized on a positive electrode by being included ina composition (e.g., a coating, a composite layer, etc.) that is appliedor deposited onto the positive electrode. In certain cases, the secondelectroactive species (e.g., polymeric or molecular electroactivematerial) infiltrates a microfiber, nanofiber, or carbon nanotube matassociated with the positive electrode, such that the secondelectroactive species is immobilized with respect to the mat of thepositive electrode. The second electroactive species may also be part ofa gel associated with the positive electrode in the same or similarmanner as described above with respect to the first electroactivespecies.

According to one or more embodiments, the electroactive composite layerof the positive electrode may have a particular ratio of weight ofelectroactive material to carbonaceous material. The ratio by weight maybe chosen to facilitate a high electrical current per mass ofelectroactive material. In some embodiments, a ratio by weight of themass of electroactive material to the mass of carbonaceous material forthe complementary electroactive composite layer may be between 1 to 2and 2 to 1. In some embodiments, it may be 1 to 1. Other ratios are alsopossible.

The separator may serve as a protective layer that can prevent therespective electrochemical reactions at each electrode from interferingwith each other. The separator may also help electronically isolate thenegative and positive electrodes from one another and/or othercomponents within the electrochemical cell to prevent short-circuiting.In some embodiments, the electrochemical cell comprises the conductivemedium and the separator contains at least a portion of the conductivemedium (e.g., conductive liquid). A person of ordinary skill, with thebenefit of this disclosure, will be able to select a suitable separator.The separator may comprise a porous structure. In some instances, theseparator is or comprises a porous solid material. In some embodiments,the separator is or comprises a membrane. The membrane of the separatormay be made of suitable material. For example, the membrane of theseparator may be or comprise a plastic film. Non-limiting examples ofplastic films included include polyamide, polyolefin resins, polyesterresins, polyurethane resin, or acrylic resin and containing lithiumcarbonate, or potassium hydroxide, or sodium-potassium peroxidedispersed therein. The material for the separator may comprise acellulose membrane, a polymeric material, or a polymeric-ceramiccomposite material. Further examples of separators includepolyvinylidene difluoride (PVDF)separators, PVDF-Alumina separators, orCelgard.

In the context of this disclosure, a conductive medium is understood tobe a solid or fluid medium having sufficient ionic conductivity tosupport the operation of an electrochemical cell (e.g., by shuttlingions between the electrodes of the electrochemical cell to maintaincharge balance). As mentioned above, the conductive medium may be aliquid or a solid electrolyte. In some embodiments, the conductivemedium is or comprises a non-volatile liquid. In some such instances,the conductive medium is or comprises a room temperature ionic liquidsuch as 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([Bmim][TF₂N]). It should be understood that while the conductive mediumcan transport ions, the conductive medium is generally notelectronically conductive (e.g., a metallic conductive) capable ofshort-circuiting an electrochemical cell when in contact with a negativeelectrode and a positive electrode.

In some cases, a separator contains a conductive liquid, which serves asthe conductive medium. In some embodiments, the separator is at leastpartially (or completely) impregnated with the conductive liquid. Forexample, the separator may absorb an amount of the conductive liquidupon being submerged, coated, dipped, or otherwise associated with theconductive liquid. In some such cases where the separator is porous,some or all of the pores of the separator (in the interior and/or nearthe surface of the separator) may become at least partially filled withthe conductive liquid. In some embodiments, the separator is saturatedwith the conductive liquid. A separator being saturated with aconductive liquid generally refers to the separator containing themaximum amount of conductive liquid capable of being contained withinthe volume of that separator at room temperature (23° C.) and ambientpressure. In some embodiments, the electrochemical cell may be providedwithout the conductive liquid present in the separator, but with theseparator capable of containing the conductive liquid when it is putinto operation to perform a gas separation process. One way in which theseparator may be capable of containing the conductive liquid is byhaving a relatively high porosity and/or containing materials capable ofabsorbing and/or being wetted by the conductive liquid.

As mentioned above, in some embodiments the conductive liquid comprisesan ionic liquid, for example, a room temperature ionic liquid (“RTIL”).The RTIL electrolyte may have a low volatility (i.e., a room temperaturevapor pressure of less than 10⁻⁵ Pa, for example, from 10⁻¹⁰ to 10⁻⁵Pa), thereby reducing the risk of electrodes drying, and allowing forflow of gas past the electrodes without significant loss to evaporationor entrainment. In some embodiments, the ionic liquid makes upsubstantially all (e.g., at least 80 vol %, at least 90 vol %, at least95 vol %, at least 98 vol %, at least 99 vol %, at least 99.9 vol %) ofthe conductive liquid.

The ionic liquid may comprise an anion component and a cation component.The anion of the ionic liquid may comprise, without limitation: halide,sulfate, sulfonate, carbonate, bicarbonate, phosphate, nitrate, nitrate,acetate, PF₆ ⁻, BF₄ ⁻, triflate, nonaflate, bis(triflyl)amide,trifluoroacetate, heptaflurorobutanoate, haloaluminate, triazolide, andamino acid derivatives (e.g. proline with the proton on the nitrogenremoved). The cation of the ionic liquid may comprise, withoutlimitation: imidazolium, pyridinium, pyrrolidinium, phosphonium,ammonium, sulfonium, thiazolium, pyrazolium, piperidinium, triazolium,pyrazolium, oxazolium, guanadinium, and dialkylmorpholinium. In someembodiments, the room temperature ionic liquid comprises an imidazoliumas a cation component. As one example, in some embodiments, the roomtemperature ionic liquid comprises 1-butyl-3-methylimidazolium (“Bmim”)as a cation component. In some embodiments, the room temperature ionicliquid comprises bis(trifluoromethylsulfonyl)imide (“TF₂N”) as an anioncomponent. In some embodiments, the room temperature ionic liquidcomprises 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([Bmim][TF₂N]).

In some embodiments, the room temperature ionic liquid comprises1-butyl-3-methylimidazolium tetrafluoroborate (BF₄) ([Bmim][BF₄]).

In some embodiments, the conductive liquid comprises a low-volatilityelectrolyte solution. For example, the conductive liquid may comprise aliquid solvent having a relatively high boiling point and dissolvedionic species therein (e.g., dissolved supporting electrolyte ions). Theliquid solvent having a relatively high boiling point may benon-aqueous. For example, the liquid solvent may compriseN,N-dimethylformamide (DMF) or the like.

In some cases, one or more electrodes of the electrochemical cellcomprises an electroactive composite layer. For example, in someembodiments, the negative electrode comprises an electroactive compositelayer (e.g., a primary electroactive composite layer). Referring to FIG.5 , negative electrode 110 comprises composite electroactive compositelayer 114 facing positive electrode 120 of electrochemical cell 500,according to certain embodiments. In certain cases, the positiveelectrode comprises an electroactive composite layer (e.g., acomplementary electroactive composite layer). For example, in FIG. 5 ,positive electrode 120 comprises electroactive composite layer 124facing negative electrode 110. The electroactive composite layer of thepositive electrode may also be referred to as complementaryelectroactive composite layer, as the electroactive species within itserves as an electron sink or electron source for the electroactivematerial of the negative electrode. In some cases, the electroactivecomposite layer of an electrode (e.g., negative electrode, positiveelectrode) extends through the entire thickness dimension of anelectrode. For example, the electroactive composite layer mayintercalate through an entire thickness of an electrode. However, insome embodiments, the electroactive composite layer of an electrode doesnot extend through the entire thickness dimension of an electrode. Insome such cases, the electroactive composite layer intercalates throughsome of but not the entire thickness of the electrode. In certain cases,the electroactive composite layer is a coating on the surface of anothercomponent of the electrode (e.g., a current collector, a gas permeablelayer, etc.).

In some embodiments, the electroactive species of an electrode (e.g.,the first electroactive species of the negative electrode, the secondelectroactive species of the positive electrode), are part of anelectroactive composite layer. For example, in FIG. 5 , electroactivecomposite layer 114 comprises the first electroactive species describedherein, according to some embodiments. Similarly, in some embodiments,electroactive composite layer 124 comprises the second electroactivespecies (e.g., polyvinylferrocene).

In addition to the electroactive species, the electroactive compositelayer of the negative electrode may also comprise a carbonaceousmaterial. Examples of suitable materials include, but are not limitedto, carbon nanotube (e.g., single-walled carbon nanotube,multi-walled-carbon nanotube), carbon black, KetjenBlack, carbon blackSuper P, graphene, or combinations thereof. Other examples also includeimmobilizing and/or coating of the electroactive species (e.g., inpolymeric forms, molecular forms or otherwise) into/onto a microfiber,nanofiber or carbon nanotube mat via intercalation, grafting, chemicalvapor deposition (CVD), or otherwise.

According to one or more embodiments, the electroactive composite layerof the negative electrode may have a particular ratio of weight ofelectroactive species to carbonaceous material. The ratio by weight maybe chosen to facilitate a high electronic current per mass ofelectroactive material. In some embodiments, a ratio by weight of themass of electroactive material to the mass of carbonaceous material maybe between 1 to 1 and 1 to 10. In some embodiments, it may be 1 to 3.Other ratios are also possible.

The negative electrode may further comprise a gas permeable layer. Thegas permeable layer (which may also be referred to as a substrate layer)may be proximate to the electroactive composite layer, and facingoutward from the electrochemical cell. In some embodiments, the gaspermeable layer is in contact with the first electroactive species. Insome such cases, the gas permeable layer is in direct contact with thefirst electroactive species, while in other such cases, the gaspermeable layer is in indirect contact with the first electroactivespecies. It should be understood that when a portion (e.g., layer,) is“on” or “in contact with” another portion, it can be directly on theportion, or an intervening portion (e.g., layer) also may be present (inwhich case the portion is understood to be “indirectly on” or “inindirect contact with” the other portion). A portion that is “directlyon”, “in direct contact with”, another portion means that no interveningportion is present. It should also be understood that when a portion isreferred to as being “on” or “in contact with” another portion, it maycover the entire portion or a part of the portion. In some embodiments,the gas permeable layer is in contact (e.g., in direct contact with orin indirect contact with) with the electroactive composite layer of thenegative electrode.

A fluid mixture in the form of a gas stream (e.g., comprising the firstLewis acid gas and the second Lewis acid gas) may diffuse through thegas permeable layer to come into contact with the electroactivecomposite layer. The gas permeable layer may comprise a conductive solidmaterial and act as a current collector within the cell.

The gas permeable layer may comprise a porous material. In someembodiments, the gas permeable layer has a porosity, for example, ofgreater than or equal to 60%, greater than or equal to 70%, greater thanor equal to the 75%, greater than or equal to 80%, or greater. In someembodiments, the gas permeable layer has a porosity of less than orequal to 85%, less than or equal to 90%, or more. Combinations of theseranges are possible. For example, in some embodiments, the gas permeablelayer of the negative electrode has a porosity of greater than or equalto 60% and less than or equal to 90%. Other porosities are alsopossible. Examples of suitable materials for the gas permeable layerinclude, without limitation, carbon paper (treated, TEFLON-treated, oruntreated), carbon cloth, and nonwoven carbon mat. Other materials mayalso be used.

While in some embodiments the electrochemical cell comprises a singlenegative electrode, in other embodiments the electrochemical cellcomprises more than one negative electrode. For example, in someembodiments, the negative electrode described herein is a first negativeelectrode, and the electrochemical cell comprises a second negativeelectrode. The positive electrode may be between the first negativeelectrode and the second negative electrode. The second negativeelectrode may also comprise the first electroactive species. The secondnegative electrode may be identical in configuration and composition tothe first negative electrode. In some embodiments, the electrochemicalcell comprises greater than or equal to 1 negative electrode, greaterthan or equal to 2 negative electrodes, greater than or equal to 3negative electrodes, greater than or equal to 5 negative electrodes,greater than or equal to 10 negative electrodes, and/or up to 15negative electrodes, up to 20 negative electrodes, up to 50 negativeelectrodes, or more.

While in some embodiments the electrochemical cell comprises a singleseparator (e.g., between the negative electrode and the positiveelectrode), in other embodiments the electrochemical cell comprises morethan one separator. For example, in some embodiments, the separatordescribed herein is a first separator, and the electrochemical cellcomprises a second separator. In some embodiments in which a secondnegative electrode is present, the second separator may be between thepositive electrode and the second negative electrode. The secondseparator may be identical in configuration and composition to the firstseparator. In certain cases, the second separator is capable ofcomprising (e.g., being saturated with) the conductive liquid. In someembodiments, the electrochemical cell comprises greater than or equal to1 separator, greater than or equal to 2 separators, greater than orequal to 3 separators, greater than or equal to 5 separators, greaterthan or equal to 10 separators, and/or up to 15 separators, up to 20separators, up to 50 separators, or more. In some cases, each of theseparators is between a respective negative electrode and positiveelectrode.

In some embodiments of the electrochemical cell in which the positiveelectrode has a negative electrode on either side (e.g., a firstnegative electrode and a second negative electrode), the positiveelectrode comprises second electroactive species facing each of thenegative electrodes. In some such embodiments, the positive electrodecomprises two complementary electroactive composite layers, each facingone of the negative electrodes.

The positive electrode may further comprise a substrate layer positionedproximate to or between the electroactive composite layer or layers. Thesubstrate layer may be in direct contact or in indirect contact with theelectroactive composite layer or layers. The substrate layer of thepositive electrode may comprise the same or different material as thatof the substrate layer of the negative electrode (when present). Forexample, the substrate layer may comprise a material such as carbonpaper (treated, TEFLON-treated, or untreated), carbon cloth, or nonwovencarbon mat. The substrate may comprise, in some embodiments, a matcomprising, for example carbon nanotubes, microfibers, nanofibers, orcombinations thereof. Other materials are also possible. The substratelayer of the positive electrode may comprise a conductive material andact as a current collector within the cell. In some embodiments, thesubstate comprises a metal and/or metal alloy. For example, thesubstrate may comprise a metal and/or metal alloy foil (e.g., having arelatively small thickness of less than or equal to 200 microns, lessthan or equal to 100 microns, less than or equal to 10 microns, and/oras low as 1 micron, or less). Examples of suitable foils could include,but are not limited to, aluminum foils, titanium foils. As a particularexample, in some embodiments, the positive electrode comprises asubstrate between a first complementary electroactive composite layerfacing the first negative electrode and a second complementaryelectroactive composite layer facing the second negative electrode. Inthis context, an electroactive composite layer of the positive electrodecan be facing a particular electrode (e.g., a negative electrode) if aline extending away from the bulk of the electroactive composite layercan intersect that electrode without passing through the substrate. Anobject (e.g., electroactive composite layer) can be facing anotherobject when it is in contact with the other object, or when one or moreintermediate materials are positioned between the surface and the otherobject. For example, two objects that are facing each other can be incontact or can include one or more intermediate materials (e.g., aseparator) between them.

FIG. 6 depicts a schematic cross-sectional diagram of an example of anelectrochemical cell, according to some, but not necessarily allembodiments, and having one or more of the components described above.Electrochemical cell 600 comprises a positive electrode 120 between twonegative electrodes 110. Separators 130 separate positive and negativeelectrodes 120 and 110. Each of negative electrodes 110 comprises anoptional gas permeable layer 112, which is positioned away from thecenter of the cell 100, and an optional primary electroactive compositelayer 114, which faces toward the positive electrode 120. In someembodiments, positive electrode 120 comprises substrate layer 122 andtwo complementary electroactive composite layers 124 thereon. Thedifferent components of the electrochemical cell 100 may have certainproperties described throughout this disclosure, for example, comprisingthe electrode materials (e.g., electroactive species) described above.The configuration of two outwardly-facing negative electrodes 110, asshown, for example, in FIG. 2 , may, in some cases, provide theadvantage of doubling the gas-adsorbing area exposed to the gas comparedto electrochemical cells comprising a single negative electrode and asingle positive electrode. The electrochemical apparatus can be providedin any of a variety of forms, depending on a desired application and/orthe nature of the fluid mixture. The electrochemical apparatus may beconfigured to electrochemically capture and/or separate Lewis acid gasesfrom gas mixtures. In some such instances, the electrochemical apparatuscomprises a chamber with a gaseous or vacuum headspace able to be filledat least partially with the gaseous fluid mixture. In some suchembodiments, the fluid inlet of the chamber is fluidically connected toa source of the gas mixture and one or more components for causing thegas mixture to be transported, such as a pump or vacuum and associatedvalving.

In some embodiments, the electrochemical apparatus is configured toelectrochemically capture and/or separate Lewis acid gases from liquidmixtures. In some such instances, the electrochemical apparatuscomprises a chamber able to be at least partially filled with asolution. In certain instances, the electrochemical apparatus, includingthe chamber and the electrochemical cell, is configured like that of aredox flow battery, wherein one of the flowed liquid solutions entersvia the fluid inlet of the chamber and exits via the fluid outlet duringoperation. In certain embodiments, a portion of the chamber in fluidiccontact with the negative electrode is fluidically connected to anabsorbent material. As one non-limiting example, the chamber may befluidically connected to an absorber tower. However, in someembodiments, the electrochemical apparatus is configured such that thefirst Lewis acid gas is captured directly at the negative electrode(e.g., by binding with the electroactive species during and/or after theapplication of the potential difference).

In some embodiments, the electrochemical cell is configured as asolid-state electrochemical cell system. In some such instances, theelectroactive species may be immobilized on at least part of thenegative electrode, as described above.

The electrochemical apparatus may be configured as a gas separationsystem. According to one or more embodiments, one or moreelectrochemical cells as described herein (e.g., configured forselective removal of Lewis acid gases) may be incorporated into a gasseparation system. The gas separation system may comprise a plurality ofelectrochemical cells, according to any of the embodiments describedherein, in fluid communication with a gas inlet and a gas outlet. Theelectrochemical cells electrically connected in parallel or in series,as described in more detail below.

The gas separation system may comprise an external circuit connectingthe negative electrode (or the first and second negative electrodes whenboth are present) and the positive electrode of each electrochemicalcell to a power source configured to apply a potential difference acrossthe negatives electrode(s) and the positive electrode of eachelectrochemical cell.

FIG. 7A shows a schematic drawing of an exemplary system performing agas separation process during a charge mode, according to one or moreembodiments. In FIG. 7A, a potential difference is applied across eachof electrochemical cells 700, such that each operates in a charge mode,according to certain embodiments. In the charge mode, a redox reaction(e.g., reduction) of the first electroactive species in the negativeelectrode 710 increases the affinity between the electroactive speciesand Lewis acid gas 790, according to certain embodiments. A gas mixture775 comprising the Lewis acid gas 790 is introduced to the system andpasses in proximity to the negative electrodes 710. The increasedaffinity causes the Lewis acid gas (e.g., SO₂) to bond to theelectroactive material, according to certain embodiments. In thismanner, at least a portion of the Lewis acid gas is separated from thegas mixture 775 to produce treated gas mixture 785.

In some embodiments, a gas separation system comprises a plurality ofelectrochemical cells, and a flow field is between at least some (e.g.,some or all) of the plurality of electrochemical cells. As anillustrative example, FIG. 7B shows a schematic drawing of an exemplarysystem comprising flow fields 711 separating electrochemical cells 570,performing a gas separation process during a charge mode, according toone or more embodiments. It should be understood that when a firstobject is between a second object and a third object, it may be betweenan entirety of the first object and second object or between portions ofthe first object and second object. In some embodiments, a flow fieldbetween two neighboring electrochemical cells is directly adjacent toeach of the neighboring electrochemical cells such that no interveningstructures/layers are between the flow field and the electrochemicalcells. However, in some embodiments, a flow field between twoneighboring electrochemical cells is indirectly adjacent to one or bothcells, such that there are one or more intervening structures/layerssuch as electrically conductive solids.

A flow field generally refers to a solid structure configured to definepathways through which a fluid may flow. In some instances, a flow fieldcomprises a solid article defining pores or channels for fluid flowwhile allowing the fluid to be exposed to adjacent structures. Suitablematerials for the solid articles of flow fields include, but are notlimited to, polymeric materials (e.g., plastics), metals/metal alloys,graphite, composite materials (e.g., a graphite-polymer composite). Insome embodiments, a flow field comprises a solid article comprising oneor more surfaces with patterned channels. The channel patterns may beselected to distribute fluid (e.g., gas) effectively across one or moredimensions of the flow field. Suitable channel patterns include, but arenot limited to serpentine, parallel, and interdigitated. FIGS. 7C, 7D,and 7E show side-view schematic drawings of faces of flows field 711 ahaving a serpentine pattern, flow field 711 b having a parallel pattern,and flow field 711 c having an interdigitated pattern, respectively withfluid flow direction indicated as arrows, according to certainembodiments. Flow field channel patterns can be formed, for example, viaetching, cutting, stamping, molding, milling, or additive manufacturing.In some embodiments, a flow field comprises a porous solid. For example,a flow field may comprise carbon fiber paper, felt, or cloth, or metalfoam.

In FIG. 7B, Lewis acid gas 790 from fluid mixture 775 is distributedalong a facial area of electrode 710 via flow field 711 (e.g., viachannels not shown). It has been realized in the context of the presentdisclosure that flow fields may assist with distributing gas mixturesrelatively uniformly across electrodes and may assist with controllingthe duration of exposure of the gas to the electrodes (e.g., to promoteefficient capture of target gases). Relatively uniform distribution ofgas may increase efficiency by utilizing a larger percentage ofelectrode area (e.g., comprising electroactive species in at least onereduced state) for binding target gas. In some embodiments, during atleast a portion of a charging process, a flux of the gas mixture acrossat least 25%, at least 50%, at least 75%, at least 90%, at least 95%, ormore of a facial area of a negative electrode in the system is within50%, within 25%, within 15%, within 10%, within 5%, within 2%, within 1%or less of an average flux across the entire facial area of the negativeelectrode during the charging process.

As mentioned above, a gas separation system may comprise a plurality ofelectrochemical cells electrically connected in parallel or in series.One of ordinary skill in the art, with the benefit of this disclosure,would understand generally how to electrically connect electrochemicalcells to form a circuit. Such connections can be made by establishing anelectrically conductive pathway for electrons to flow between electrodesof the electrochemical cells (in other words, establishing electricalcoupling between electrodes). An electrically conductive pathway may insome instances be established via one or more electrically conductivesolid materials (e.g., conductive metals, alloys, polymers, composites,carbonaceous materials, or combinations thereof). For example, anelectrically conductive pathway may be established via wiring electrodesof the electrochemical cells. The electrochemical cells may have any ofthe configurations described above. For example, in some embodiments,some or all of the electrochemical cells in the system have a singlenegative electrode (e.g., comprising a first electroactive species), asingle positive electrode (e.g., comprising a second electroactivespecies), and optionally a separator between the first positiveelectrode and the second positive electrode. FIG. 8A shows a schematicdrawing of an arrangement of electrochemical cells 1100 in one suchsystem 1000, where each electrochemical cell 1100 comprises, in order,negative electrode 1010, optional separator 1020, and positive electrode1030, according to certain embodiments. A gas mixture 1075 comprising atarget gas may be introduced to the system such that gas mixture 1075passes in proximity to negative electrode 1010 of first electrochemicalcell 110 and positive electrode 1030 of neighboring secondelectrochemical cell 1100. While FIG. 8A shows three electrochemicalcells 1100, it should be understood than any of a variety of suitablenumbers of electrochemical cells may be employed in a gas separationsystem (e.g., electrically connected in parallel or in series),depending on the requirements of a particular application as needed.

In other embodiments, some or all of the electrochemical cells in a gasseparation system comprise a positive electrode (e.g., comprising asecond electroactive species), a first negative electrode (e.g.,comprising the first electroactive species), a second negative electrode(e.g., comprising the first electroactive species), a first separatorbetween the first negative electrode and the positive electrode, and asecond separator between the positive electrode and the second negativeelectrode. Examples of such electrochemical cells are shown in FIG. 6and FIGS. 7A-7B.

FIG. 8B shows a schematic drawing of configuration in which a pluralityof electrochemical cells 1100 in system 1000 are electrically connectedin parallel, according to certain embodiments. In a parallelconfiguration, each negative electrode 1010 is electrically coupled to afirst terminal (e.g., of a power source) and each positive electrode1030 is electrically coupled to a second terminal (e.g., of a powersource). For example, in FIG. 8B, each negative electrode 1010 iselectrically coupled to a first terminal of a power source via wiring115, and each positive electrode 1030 is electrically coupled to asecond terminal of the power source via wiring 116, in accordance withcertain embodiments.

FIG. 8C shows a schematic drawing of a configuration in which aplurality of electrochemical cells 11000 in system 1000 are electricallyconnected in series, according to certain embodiments. In a seriesconfiguration, a positive electrode of a first electrochemical cell iselectrically connected to a negative electrode of a secondelectrochemical cell of the system. For example, in FIG. 8B, negativeelectrode 1010 of first electrochemical cell 1100 a is electricallyconnected to positive electrode 1030 of second electrochemical cell 1100b via wiring 1017, and negative electrode 1010 of second electrochemicalcell 1100 b is electrically connected to positive electrode 1030 ofthird electrochemical cell 1100 c via wiring 1018, according to certainembodiments. Further, positive electrode 1030 of first electrochemicalcell 1100 a is electrically coupled to a first terminal of a powersource via wiring 114, and negative electrode 1030 of thirdelectrochemical cell 1100 a is electrically coupled to a second terminalof the power source via wiring 119, in accordance with certainembodiments.

It has been determined in the context of this disclosure that certainconfigurations of gas separation systems comprising a plurality ofelectrochemical cells electrically connected in series may promoterelatively efficient charge transport/and/or gas transport. For example,in some embodiments, electrically conductive materials betweenelectrochemical cells may establish electrically conductive pathwaysrather than using external wiring. For example, a gas separation systemmay comprise a first electrochemical cell and a second electrochemicalcell electrically connected in series, where the electrical connectionis established via one or more electrically conductive materials betweenthe first electrochemical cell and the second electrochemical cell. Anyof a variety of suitable electrically conductive materials may bepositioned between electrochemical cells to establish electricalconnection between, for example, a negative electrode of the firstelectrochemical cell and a positive electrode of the secondelectrochemical cell. For example, an electrically conductive materialmay be an electrically conductive solid. The electrically conductivesolid may comprise, for example, a metal and/or metal alloy (e.g.,steel, silver metal/alloy, copper metal/alloy, aluminum metal/alloy,titanium metal/alloy, nickel metal/alloy). In some embodiments, theelectrically conductive solid comprises a carbonaceous material (e.g.,graphite, single-walled carbon nanotubes, multi-walled-carbon nanotubes,carbon black, a carbon mat (e.g., carbon nanotube mat), KetjenBlack,carbon black Super P, graphene, and the like. In some embodiments, thecarbonaceous material is a porous carbonaceous material as describedelsewhere herein. In some embodiments, the electrically conductive solidcomprises a composite of an electrically conductive solid with a binderresin. In some embodiments, an electrically conductive solid betweenelectrochemical cells comprises an electrically conductive polymericmaterial.

In some, but not necessarily all embodiments, an electrically conductivematerial between electrochemical cells comprises a bipolar plate. Itshould be understood that in the context of this disclosure a plate neednot necessarily be flat. Bipolar plates are known to those of skill inthe art and are typically used in fields other than gas separation, suchas in fuel cells. A bipolar plate may be configured to separate fluid(e.g., gas) contacting the positive electrode from the fluid contactingthe negative electrode. Bipolar plates may comprise electricallyconductive solids such as steel, titanium, or graphite.

In some embodiments, at least some of the plurality of electrochemicalcells (e.g., connected in series) are separated by a flow field. Asmentioned above, positioning a flow field between neighboringelectrochemical cells may promote beneficial gas distribution andrelatively efficient interaction between gases and the electrodes (e.g.,for binding). In some embodiments, a bipolar plate as described abovecomprises a flow field (e.g., via etching of fluidic pathways in one orboth faces of the plate), though in other embodiments a different flowfield is employed as an alternative or in addition to theflow-field-containing bipolar plate.

FIG. 9 shows a schematic diagram of exemplary gas separation system 1000comprising electrochemical cells 1100 electrically connected in seriesvia one or more electrically conductive materials between cells,according to certain embodiments. In FIG. 9 , system 1000 compriseselectrically conductive solid materials in the form of bipolar plates1012 and ribs 1014. Ribs in a gas separation system may be made of anyof the electrically conductive solid materials described above. In theembodiment shown in FIG. 9 , first electrochemical cell 1100 a isseparated from second electrochemical cell 1100 b via bipolar plate 1012and rib 1014. Bipolar plate 1012 and rib 1014 may be directly adjacentto negative electrode 1010 of first electrochemical cell 1100 a andpositive electrode 1030 of second electrochemical cell 1100 b, therebyestablishing an electrically conductive pathway for the seriesconnection. Other electrochemical cells in the system may beelectrically connected similarly. While FIG. 9 shows bipolar plates andribs, such a depiction is non-limiting, and other configurations (e.g.,without bipolar plates, without ribs, etc.) are possible. FIG. 9 alsoshows optional flow fields 1011 separating electrochemical cells 1100,in accordance with certain embodiments. In some embodiments, one or morecomponents (e.g., electrically conductive solids such as ribs) mayestablish channels between negative electrodes and positive electrodesof neighboring electrochemical cells. For example, ribs 1014 in FIG. 9may have dimensions such that channels 1013 establish pathways for gas(e.g., gas mixtures) to flow between electrochemical cells 1011 andinteract with the electrodes. For example, gas mixture 1075 may bepassed through channel 1013, through flow field 1011, and between firstelectrochemical cell 1100 a and second electrochemical cell 1100 b,according to certain embodiments.

The flow of electrical current in certain embodiments described abovemay encounter less electrical resistance compared to otherconfigurations. For example, in some embodiments in whichelectrochemical cells are connected in series via electricallyconductive materials between at least some of a stack of electrochemicalcells, electrical current can flow in a direction perpendicular to thestack. FIG. 9 shows one such example, where electrical current can flowin direction x perpendicular to electrochemical cells 1100, while gasmixture 1075 can flow in a direction parallel to electrochemical cells1100. In FIG. 9 , the path through which the current travels isrelatively short and is determined by the thickness of bipolar plate1012 and rib 1014. In some embodiments, a thickness of the one or moreelectrically conductive solids between electrochemical cells is lessthan or equal to 10 mm, less than or equal to 5 mm, less than or equalto 2 mm, less than or equal to 1 mm, and/or as low as 0.5 mm, as low as0.2 mm, as low as 0.1 mm, or lower. In contrast, in embodiments in whichelectrochemical cells are electrically connected in parallel orelectrically connected in series via external wiring, electrical currentmust flow through up to an entire height and/or length of electrodes(e.g., current collectors of electrodes) and through electrode tabs toreach the external wiring. Such heights and/or lengths may be, forexample, at least 1 cm, at least 2 cm, at least 5 cm, at least 10 cm,and/or up to 20 cm, up to 50 cm, up to 100 cm, or more. The greaterdistances for current travel in such embodiments generally results ingreater total cell resistance, which may reduce charge transport and/orenergy efficiency for methods of at least partial gas separationdescribed herein.

In some embodiments, the negative electrode or portion thereof (e.g., anelectroactive composite layer of the negative electrode when present) isbe able to absorb a gas (e.g., SO₂, CO₂) at a particular rate. Forexample, in some embodiments, the negative electrode or portion thereof(e.g., an electroactive composite layer of the negative electrode whenpresent) has an absorption capacity rate of at least 0.0001 mol per m²per second, at least 0.0002 mol per m² per second, at least 0.0005 molper m² per second, or more. In some embodiments, the negative electrodeor portion thereof (e.g., an electroactive composite layer of thenegative electrode when present) has an absorption capacity rate of lessthan or equal to 0.001 mol per m² per second, less than or equal to0.0008 mol per m² per second, less than or equal to 0.0005 mol per m²per second, or less. In some embodiments, the electroactive compositelayer has an absorption capacity rate of at least 0.0001 and less thanor equal to 0.0005 mol per m² per second. Other absorption capacitiesrates are also possible.

In some embodiments, an electroactive composite layer of a negativeelectrode may have a particular surface area able to be exposed to afluid mixture (e.g., gas mixture), for example, of greater than or equalto 5 cm², greater than or equal to 8 cm², greater than or equal to 10cm², and/or up to 10 cm², up to 20 cm², up to 50 cm², up to 1 m², ormore. Other values are also possible.

In some embodiments, at least a portion or all of an electrode (e.g.,negative electrode, positive electrode) described herein is comprises aporous material. A porous electrode may be made of any suitable materialand/or may comprise any suitable shape or size. In a non-limitingembodiment, the electrode comprises a porous carbonaceous material. Theterm carbonaceous material is given its ordinary meaning in the art andrefers to a material comprising carbon or graphite that is electricallyconductive. Non-limiting example of carbonaceous materials includecarbon nanotubes, carbon fibers (e.g., carbon nanofibers), carbon mat(e.g., carbon nanotube mat), and/or graphite. In some such embodiments,the electrode may be partially fabricated from the carbonaceous materialor the carbonaceous material may be deposited over an underlyingmaterial. The underlying material generally comprises a conductivematerial, for example, a metal and/or metal alloy solid (e.g., steel,copper, aluminum, etc.). Other non-limiting examples of conductivematerials are described herein.

In some embodiments, an electrode (e.g., the negative electrode, thepositive electrode) is porous. The porosity of an electrode may bemeasured as a percentage or fraction of the void spaces in theelectrode. The percent porosity of an electrode may be measured usingtechniques known to those of ordinary skill in the art, for example,using volume/density methods, water saturation methods, waterevaporation methods, mercury intrusion porosimetry methods, and nitrogengas adsorption methods. In some embodiments, the electrode is at least10% porous, at least 20% porous, at least 30% porous, at least 40%porous, at least 50% porous, at least 60% porous, at least 70% porous orgreater. In some embodiments, the electrode is up to 90% porous, up to85% porous, up to 80% porous, up to 70% porous, up to 50% porous, up to30% porous, up to 20% porous, up to 10% porous or less. Combinations ofthese ranges are possible. For example, the electrode may be at least10% porous and up to 90% porous. The pores may be open pores (e.g., haveat least one part of the pore open to an outer surface of the electrodeand/or another pore). In some cases, only a portion of the electrode isporous. For example, in some cases, only a single surface of theelectrode is porous. As another example, in some cases, the outersurface of the electrode is porous and the inner core of the electrodeis substantially non-porous (e.g., less than or equal to 20%, less thanor equal to 10% porous, less than or equal to 5% porous, less than orequal to 1% or less). In a particular embodiment, the entire electrodeis substantially porous.

In some embodiments, the electrochemical cell has a particular cycletime. The cycle time of an electrochemical cell generally refers to theperiod of time in performance of one charge mode and one discharge mode.The cycle time may be at least 60 seconds, at least 100 seconds, atleast 300 seconds, at least 500 seconds, at least 1000 seconds, or more.In some embodiments, the cycle time is less than or equal to 3600seconds, less than or equal to 2400 seconds, less than or equal to 1800seconds, or less. Combinations of these ranges are possible. Forexample, in some embodiments, the cycle time is at least 60 seconds andless than or equal to 3600 seconds, or at least 300 seconds and lessthan or equal to 1800 seconds.

According to some embodiments, the electrochemical cell and itscomponents have a particular thickness, depending on the desiredapplication (e.g., gas separation of ventilator air, direct air capture,etc.). In some embodiments, the electrochemical cell has a thickness ofat least 10 μm, at least 20 μm, at least 50 μm, at least 100 μm, atleast 200 μm, at least 300 μm, at least 500 μm, or greater. In someembodiments, the electrochemical cell has a thickness of less than orequal to 750 μm, less than or equal to 600 μm, less than or equal to 500μm, less than or equal to 300 μm, or less. Combinations of these rangesare possible. For example, in some embodiments, the electrochemical cellhas a thickness of at least 200 μm and less than or equal to 750 μm. Insome embodiments, the electrochemical cell has a thickness of at least10 μm and less than or equal to 750 μm.

In some embodiments, the negative electrode or the positive electrodehas a thickness of at least 0.5 μm, at least 1 μm, at least 2 μm, atleast 5 μm, at least 10 μm, at least 20 μm, at least 50 μm, at least 75μm, at least 100 μm or more. In some embodiments, the negative electrodeor the positive electrode has a thickness of less than or equal to 200μm, less than or equal to 150 μm, less than or equal to 100 μm, or less.Combinations of these ranges are possible. For example, in someembodiments, the negative electrode or the positive electrode has athickness of at least 50 μm and less than or equal to 200 μm. In someembodiments, in some embodiments, the negative electrode or the positiveelectrode has a thickness of at least 0.5 μm and less than or equal to200 μm.

In some embodiments, the electroactive composite layer of the negativeelectrode or the positive electrode has a thickness of at least 10 nm,at least 20 nm, at least 40 nm, at least 0.1 μm, at least 0.2 μm, atleast 0.5 μm, at least 1 μm, at least 2 μm, at least 5 μm, at least 10μm, at least 50 μm, at least 100 μm or more. In some embodiments, theelectroactive composite layer of the negative electrode or the positiveelectrode has a thickness of less than or equal to 200 μm, less than orequal to 150 μm, less than or equal to 100 μm, less than or equal to 50μm, less than or equal to 20 μm, less than or equal to 10 μm, less thanor equal to 5 μm, less than or equal to 2 μm, less than or equal to 1μm, less than or equal to 0.5 μm, less than or equal to 0.2 μm, lessthan or equal to 0.1 μm, or less. Combinations of these ranges arepossible. For example, in some embodiments, the electroactive compositelayer of the negative electrode or a positive electrode has a thicknessof greater than or equal to 10 μm and less than or equal to 200 μm. Insome embodiments, the electroactive composite layer of the negativeelectrode or a positive electrode has a thickness of greater than orequal to 10 nm and less than or equal to 100 nm, or greater than orequal to 50 nm and less than or equal to 500 nm.

Various components of a system, such as the electrodes (e.g., negativeelectrode, positive electrodes), power source, electrolyte, separator,container, circuitry, insulating material, etc. can be fabricated bythose of ordinary skill in the art from any of a variety of components.Components may be molded, machined, extruded, pressed, isopressed,infiltrated, coated, in green or fired states, or formed by any othersuitable technique. Those of ordinary skill in the art are readily awareof techniques for forming components of system herein.

The electrodes described herein (e.g., negative electrode, positiveelectrodes) may be of any suitable size or shape. Non-limiting examplesof shapes include sheets, cubes, cylinders, hollow tubes, spheres, andthe like. The electrodes may be of any suitable size, depending on theapplication for which they are used (e.g., separating gases fromventilated air, direct air capture, etc.). Additionally, the electrodemay comprise a means to connect the electrode to another electrode, apower source, and/or another electrical device.

Various electrical components of system may be in electricalcommunication with at least one other electrical component by a meansfor connecting. A means for connecting may be any material that allowsthe flow of electricity to occur between a first component and a secondcomponent. A non-limiting example of a means for connecting twoelectrical components is a wire comprising a conductive material (e.g.,copper, silver, etc.). In some cases, the system may also compriseelectrical connectors between two or more components (e.g., a wire andan electrode). In some cases, a wire, electrical connector, or othermeans for connecting may be selected such that the resistance of thematerial is low. In some cases, the resistances may be substantiallyless than the resistance of the electrodes, electrolyte, and/or othercomponents of the system.

In some embodiments, the methods and electrochemical apparatusesdescribed herein can be performed and configured as one or more of thesystems described in U.S. Patent Publication No. 2017/0113182, publishedon Apr. 27, 2017, filed as application Ser. No. 15/335,258 on Oct. 26,2016, and entitled “Electrochemical Process for Gas Separation,” whichis incorporated herein by reference in its entirety for all purposes.

U.S. Provisional Application No. 62/892,975, filed Aug. 28, 2019, andentitled “Electrochemically Mediated Acid Gas Removal andConcentration,” and U.S. Provisional Application No. 62/988,851, filedMar. 12, 2020, and entitled “Electrochemical Capture of Lewis AcidGases,” are each incorporated herein by reference in its entirety forall purposes.

The following examples are intended to illustrate certain embodiments ofthe present invention, but do not exemplify the full scope of theinvention.

Example 1

This Example describes the reactivity of various electroactive specieswith Lewis acid gases as studied by cyclic voltammetry.

FIG. 10A shows cyclic voltammetry of 1,4-naphthoquinone (p-NQ) in a dryN,N-dimethylformamide solution containing 0.1 M tetra-n-butylammoniumhexafluorophosphate ([nBu₄][PF₆]) saturated with either N₂, CO₂, or SO₂.The cyclic voltammograms were acquired at a 100 mV/s scan rate. Thecyclic voltammetry in FIG. 10A showed expected behavior under N₂ andCO₂, where the second (more negative) reduction wave shifted positivelyunder CO₂ with respect to under N₂, while the first reduction wave didnot shift. However, in the presence of SO₂, the first (less negative)reduction wave, along with the second reduction wave, shiftedpositively. This was indicative of a strong association of thesemiquinone with the strong Lewis acid SO₂, which caused a major shiftin the Nernst potential. The dissociative oxidation peak of the complexof the reduced 1,4-napthquinone and the SO₂ appeared at a more positivepotential than that of the CO₂ complex, which further confirmed thestrong association of SO₂ with the semiquinone, as well as the quinonedianion. The differences in the cyclic voltammograms indicated differentstrengths of association between the reduced 1,4-naphthoquinone and SO₂and CO₂.

These cyclic voltammetry results demonstrated that certain electroactivespecies such as 1,4-naphthoquinone react strongly with both SO₂ and CO₂,such that exposure to a mixture of both Lewis acid gases would result inbinding with limited to no selectivity. Such a lack of selectivity couldbe problematic in certain applications such as electrochemical flowsystems for carbon dioxide capture where gas mixtures comprise both CO₂and SO₂, as SO₂ capture would diminish CO₂ capture efficiency. Thedifference in the strengths of association of 1,4-naphthoquinone withCO₂ and SO₂ implied that in a flow system for carbon capture fromindustrial exhaust, where oxides of sulfur are present at concentrationsof 1,000-10,000 ppm, the “poisoning” of the quinone is quite possible.It is believed that this would mainly be due to the difference in theoxidation potential, where an electrochemical cell operating at apotential difference of around the difference between the reduction ofquinone and the oxidation of its complex with CO₂ would not providesufficient energy to dissociate the quinone-SO₂ complex. This leads tothe accumulation of the complex and the subsequent decrease of thesystem capacity for CO₂.

It was realized that this lack of selectivity could be overcome byintroducing an electrochemical desulfurization step which removes theoxides of sulfur from the gas mixture (e.g., flue gas) prior to theelectrochemical carbon capture step. Such an electrochemicaldesulfurization step would involve determining electroactive speciesand/or conditions for selectively capturing sulfur dioxide (or otherLewis acid gases) while allowing carbon dioxide capture to benegligible. Toward that end, it was further realized that a similar flowsystem could be designed with an electroactive species (e.g., quinone)that has lower electron density on the oxygens upon reduction to yield acomparatively weaker base than 1,4-naphthoquinone.

This was accomplished by 2,3-dicyano-1,4-naphthoquinone (DCNQ). FIG. 10Bshows cyclic voltammetry of DCNQ in a dry N,N-dimethylformamide solutioncontaining 0.1 M [nBu₄][PF₆] saturated with either N₂, CO₂, or SO₂. Thecyclic voltammograms were acquired at a 100 mV/s scan rate. The cyclicvoltammetry in FIG. 10B showed the behavior under CO₂ as being notdissimilar from that under N₂, where only the dianion (formed upon thesecond reduction) reacted weakly with CO₂. This weak interaction wasdemonstrated by the slight positive shift of the second reduction wave,and the emergence of a very small oxidative dissociation peak.Nevertheless, the interaction of DCNQ was very strong with SO₂, aconclusion supported by the positive shift of the two reduction wavesand the emergence of a very positively-shifted dissociative oxidationpeak in FIG. 10B. These results demonstrated that certain electroactivespecies (e.g., with certain substituents) have reduced states in whichthey are capable of reacting with a first Lewis acid gas (e.g., SO₂) butfor which a reaction with a second Lewis acid gas (e.g., CO₂) is notthermodynamically favorable.

Example 2

This Example describes the reactions of various electroactive specieswith Lewis acids in ionic liquids (IL).

The reactions of DCNQ and p-NQ with SO₂ were studied bythermogravimetric analysis (TGA). DCNQ was reduced using two equivalentsof cobaltocene to yield a 0.3 M solution of DCNQ²⁻ dianion in[bmim][TF₂N]. This was used, along with the p-NQ²⁻ dianion and theNQ^(⋅−) semiquinone ionic liquid (IL) solutions in the TGA with a flowof 30 mL min⁻¹ of 1% SO₂ with the balance being N₂. FIG. 11A shows TGAanalyses of DCNQ²⁻, NQ^(⋅−) and NQ²⁻ under 1% SO₂. FIG. 11B shows TGAanalyses of DCNQ²⁻, NQ^(⋅−) and NQ²⁻ in N₂. It could be seen from FIG.11A and FIG. 11B that both NQ²⁻ and DCNQ²⁻ effectively andstoichiometrically reacted with SO₂ to form a stable diadduct which didnot release under pure N₂ flow. The capacity of NQ^(⋅−) for SO₂ wassmaller than NQ²⁻ as expected, but did not release it at the same rateit releases CO₂ under pure N₂ since the pK_(a) of NQ^(⋅−) is similar tothat of DCNQ²⁻ which is sufficiently high to react with a strong Lewisacid like SO₂ with a low pK_(a). This resulted in a stronger bondformation upon the sulfonation of the stronger Lewis base NQ²⁻ whichmaintained a constant capacity for SO₂ at high temperatures (up to 150°C.) and did not release SO₂ under pure N₂ flow, as seen in FIGS. 11C and11D. FIG. 11C shows TGA measurements of SO₂ uptake by NQ²⁻ at differenttemperatures. FIG. 11D shows TGA measurements of SO₂ uptake release byNQ²⁻ at different temperatures. Nevertheless, the lower basicity ofDCNQ²⁻ resulted in weaker sulfonation and subsequently a smallercapacity for SO₂ at higher temperatures. FIG. 11E shows TGA measurementsof SO₂ uptake by DCNQ²⁻ at different temperatures. FIG. 11F shows TGAmeasurements of SO₂ release by DCNQ²⁻ at different temperatures.

It is believed that the relative pK_(a)s of DCNQ²⁻ and NQ²⁻ explain, atleast in part, their reaction extent with CO₂ and SO₂, and are also theresult of the electron density modulation on the nucleophilic oxygen(phenoxide) moieties, which are generated upon the first and secondreduction of quinones. Thus, electroactive species such as quinones withfinely tuned electron density can be used to selectively react withdifferent electrophiles with varying Lewis base strengths in customizedelectrochemical systems where continuous separation can be performed.

In addition to electron density modulation on the quinone molecule,which directly affects the thermodynamics of the electrochemicalreactions, it has been realized in the context of the present disclosurethat it is possible to impart further selectivity to the nucleophilegenerated via steric hindrance or affinity. This could be done byattaching various groups around the nucleophilic centers to accommodatethe reductive addition of one target more favorably than others.

FIG. 12A shows TGA measurements of capture of CO₂ with reduced DCNQunder 100% CO₂ at 30° C. FIG. 12B shows TGA measurements showing thatCO₂ is released when the reactant is removed. Thus, reduced DCNQ reactswith CO₂ reversibly, but with SO₂ irreversibly.

Example 3

This Example describes a computational analysis of DCNQ to gain insightinto their electronic structure and thermodynamic properties as theyrelated to their reactivity with Lewis Acid gases.

Calculations on the neutral, singly-reduced (semiquinone) anddoubly-reduced (dianion) states of DCNQ were made using densityfunctional theory methods. All electronic structure calculations wereperformed with Q-Chem® version 5.1.1. Equilibrium structures weredetermined at the B3LYP-D3(op)/6-31++G** level of theory, withspin-unrestricted wave functions, and Grimme dispersion corrections withthe optimized power approach corrections of Witte J, Mardirossian N,Neaton J B, Head-Gordon M. Assessing DFT-D3 Damping Functions AcrossWidely Used Density Functionals: Can We Do Better? Journal of ChemicalTheory and Computation. 2017; 13(5):2043-2052, which is incorporated byreference herein in its entirety for all purposes. 1,4-naphothoquinone(Q) was treated as a neutral singlet, its semiquinone anion (Q−) andCO₂-adduct anion were treated as −1 doublet (QCO₂−), and the dianionQ²⁻, single adduct Q(CO₂)²⁻ and di-adduct Q(CO₂)₂ ²⁻ were also taken assinglets. Geometry optimizations in gas phase and solvated environments(within the SMD solvent SCRF) were performed on structures built usingthe Avogadro® computer program after optimization with the MMFF94force-field. A systematic rotor search was performed to identify lowlying conformers of QCO₂ ⁻, QCO₂ ²⁻ and Q(CO₂)₂ ²⁻.

Ground state binding energies were calculated by subtracting the totalelectronic energy of the optimized isolated species from the optimizedcomplex, including zero point energy (ZPE) and thermal and solvationcontributions. Basis set superposition error (BSSE) was accounted for bythe counterpoise scheme. Frequency analysis was used to confirm groundstate structures were a minimum on the potential energy surface. Naturalbond orbital (NBO) partial charges and orbital characteristics wereobtained with the NBO v6.0 package interfaced with Q-Chem® and secondgeneration ALMO-EDA was performed within Q-Chem®. Reduction potentialswere determined by the procedure suggested by Isse A A, Gennaro A.Absolute Potential of the Standard Hydrogen Electrode and the Problem ofInterconversion of Potentials in Different Solvents. Journal of PhysicalChemistry B. 2010; 114(23):7894-7899, which is incorporated by referenceherein in its entirety for all purposes. For the reduction potentialcalculations, structures were optimized in the solvent SCRF (withparameters for N,N-dimethylformamide), the electron free energy wasdetermined by Fermi-Dirac statistics with 4.28 V taken as the absolutevalue for the Standard Hydrogen Electrode, and junction potentials wereadjusted for with data from Diggle J W, Parker A J. Liquid junctionpotentials in electrochemical cells involving a dissimilar solventjunction. 1974 p. 1617-1621, which is incorporated by reference hereinin its entirety for all purposes.

FIGS. 13A-13C show the computed geometry changes of DCNQ upon reduction.Standard reduction potentials for the reductions of DCNQ were calculatedat standard states, as follows:

${{\Delta G_{{reducti}on}^{*}} = {G_{red} - ( {G_{ox} + G_{e^{-}}} )}}{{E^{0}({absolute})} = {- \frac{\Delta G_{{reducti}on}^{*}}{F}}}{{E^{0}( {{vs}{Ag}^{+}} )} = {{E^{0}({absolute})} + {E( {{SHE}{vs}{absolute}} )} + {E( {{SCE}{vs}{SHE}} )} + {E( {{DMF}/H_{2}O{junction}} )} + {E( {{Ag}^{+}{vs}{SCE}} )}}}$where SHE is the standard hydrogen electrode reference and SCE is thestandard calomel reference. The calculated standard reduction potentialsfor DCNQ were calculated to be −0.02 and −1.52 vs. Ag+, respectively.With the same method, the reduction potentials for 1,4-napthoquinonewere −1.27 V and −2.48 V vs Ag⁺ for the first and second reductions,respectively.

Electrostatic potential (ESP) maps for the different states of reductionof DCNQ and 1,4-naphthoquinone were also calculated. FIG. 14A shows ESPmaps of 2,3-dicyano-1,4-naphthoquinone (top) and 1,4-naphthoquinone(bottom) in their respective neutral states. FIG. 14B shows ESP maps of2,3-dicyano-1,4-naphthoquinone (top) and 1,4-naphthoquinone (bottom) intheir respective semiquinone states. FIG. 14C shows ESP maps of2,3-dicyano-1,4-naphthoquinone (top) and 1,4-naphthoquinone (bottom) intheir respective dianion states. In the ESP maps, darker shadingindicates higher charge density (more electron-rich or electron poor)and lighter shading indicates lower charge density (less electron-richor electron poor). As can be seen in FIGS. 14A-14C, the chargedistributions are relatively similar between DCNQ and 1,4-naphthoquinonein the neutral (FIG. 14A) and dianion (FIG. 14C) states. However, FIG.14B shows that the oxygen moieties of 1,4-naphthoquinone in itssemiquinone state have a significantly higher electron density than dothe oxygen moieties of DCNQ in its semiquinone state. It is believedthat the electron-withdrawing effect of the nitrile substituents pullelectron density from the oxygens of DCNQ. It is further believed thatthis shift in electron density in DCNQ renders it less thermodynamicallyand/or kinetically reactive toward Lewis acids (e.g., shifting thepK_(a)) and therefore more selective for certain Lewis acids (e.g.,depending on their pK_(a)). All maps are on the same scale (imaged onvan der Waals spheres).

FIGS. 15A-15D shows calculated geometries and ESP maps of CO₂ and SO₂,respectively.

Example 4

This Example describes selective removal of an amount of a first Lewisacid gas from a fluid mixture comprising the first Lewis acid gas and asecond, different Lewis acid gas via a reduced state of an electroactivespecies. In particular, a gas mixture comprising SO₂ and CO₂ was exposedto a reduced form of DCNQ, resulting in removal of the SO₂ from thefluid mixture to a greater extent than the CO₂.

The gas separation experiment employed a packed-bed bubble columnapparatus comprising a borosilicate glass tube (12″ length, 0.23″ innerdiameter) filled with 8.6 g of 1 mm glass beads. The bubble columnapparatus was oven dried and sealed with two septa on each end. An inletneedle was inserted into the bottom of the column, and an outlet needlewas inserted into the top of the column. The column was flushed with drynitrogen gas for 10 minutes to establish an inert atmosphere inside thecolumn. All gas streams were applied to the inlet of the column atprecise flow rates using Cole-Parmer mass flow controllers. An outletstream of the bubble column apparatus was monitored for CO₂ and SO₂ gasconcentrations. FIG. 16 shows a schematic diagram of the gas separationexperiment.

The doubly reduced DCNQ species 2,3-dicyanonaphthoquinone dianion(DCNQ²⁻) was prepared by treating a solution of2,3-dicyanonaphthoquinone (DCNQ) in tetrahydrofuran (THF) with sodium(Na) metal, removing residual metal via filtration, and removing the THFvia evaporation. A reaction scheme for the preparation of DCNQ²⁻ isshown below.

Prior to the gas capture experiment, the column apparatus was filledwith 2.4 mL solution of 12 mM 2,3-dicyanonaphthoquinone dianion (DCNQ²⁻)in propylene carbonate. The filling procedure was conducted underflowing nitrogen gas to maintain an inert atmosphere in the column.After filling the column with the DCNQ²⁻/propylene carbonate solution, astream of dry nitrogen gas was introduced to the column inlet at a flowrate of 1 mL/minute to allow the system to equilibrate. The mass flowcontroller for the inlet gas was then connected to a SO₂/CO₂/N₂ gasstream (1 mole percent (mol %) SO₂, 10 mol % CO₂, 89 mol % N₂) at 1mL/min, and data logging was immediately started. To control for solventphysisorption of the Lewis acid gases, an identical procedure wasconducted with 2.4 mL of solvent (propylene carbonate) with no DCNQ²⁻present.

The concentration of CO₂ and SO₂ were measured versus the total volumeof inlet gas introduced to the system, for the physisoprtion and DCNQ²⁻chemisorption experiments. It was observed that, due to the lowsolubility of CO₂ in propylene carbonate, its breakthrough in thephysisoprtion experiment (i.e., in the absence of DCNQ²⁻) occurred at amuch earlier point than that of SO₂, which has a higher solubility inpropylene carbonate. In the chemisorption experiment (i.e., in thepresence of DCNQ²⁻), it was observed that CO₂ breakthrough occurred at alater point than in the physisorption control experiment. Integration ofthe concentration data showed about 70% stoichiometric capture of CO₂ byDCNQ²⁻ during the chemisorption experiment. This is believed to be dueto a reversible reaction of DCNQ²⁻ with CO₂ to form the adductDCNQ²⁻(CO₂)₂, as shown below.

However, CO₂ complexed with DCNQ²⁻ was replaced by SO₂ in anirreversible reaction that formed the adduct DCNQ²⁻(SO₂)₂. ThisDCNQ²⁻(SO₂)₂ adduct formation was evident in the delayed breakthrough ofSO₂ in the chemisorption experiment when compared to the physisorptionbreakthrough. The difference in reactivity was qualitatively observed.After CO₂ breakthrough but prior to SO₂ breakthrough, the entiresolution in the column was observed to have a pink color (indicative ofthe presence of DCNQ²⁻ and DCNQ²⁻(CO₂)₂ adduct) except for a smallregion near the inlet of the column that was observed to have a yellowcolor (indicative of the presence of DCNQ²⁻(SO₂)₂). At a later point intime, after SO₂ breakthrough, an entirety of the column solution wasobserved to have the yellow color, indicating the absence of CO₂ in thecolumn and presence of only DCNQ²⁻(SO₂)₂ throughout the column. FIG. 17Ashows a plot of a ratio of outlet gas concentration to inlet gasconcentration vs. time for the physisorption and chemisorptionexperiments. In FIG. 17A, curve A corresponds to CO₂ concentration inthe physisorption (propylene carbonate only) experiment, curve Bcorresponds to CO₂ concentration in the chemisorption (DCNQ²⁻ inpropylene carbonate) experiment, curve C corresponds to SO₂concentration in the physisorption (propylene carbonate only)experiment, and curve D corresponds to SO₂ concentration in thechemisorption (DCNQ²⁻ in propylene carbonate) experiment. The results inFIG. 17A show significantly earlier breakthrough for CO₂ than for SO₂,and also shows a significantly delay in SO₂ breakthrough in thechemisorption experiment than in the physisorption experiment. FIG. 17Bshows a zoomed-in view of curves A and B from FIG. 17A.

The delay in SO₂ breakthrough accounts for a stoichiometric reactionwhere all the starting DCNQ²⁻ reacted with SO₂. Reaction schemes for thebinding of SO₂ to DCNQ²⁻ and the substitution of CO₂ by SO₂ in aDCNQ²⁻(CO₂)₂ adduct are shown below.

The results of the gas separation experiment demonstrated selectivecapture of SO₂ from a gas stream of 1% SO₂, 10% CO₂ and 89% N₂. Thechemisorption capture of SO₂ by DCNQ²⁻ is believed to be only reversiblevia an oxidation reaction to afford release of the SO₂. Such anoxidation can occur electrochemically or by the use of an oxidativechemical reagent.

While several embodiments of the present invention have been describedand illustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and/or structures for performing thefunctions and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the present invention.More generally, those skilled in the art will readily appreciate thatall parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the teachings of thepresent invention is/are used. Those skilled in the art will recognize,or be able to ascertain using no more than routine experimentation, manyequivalents to the specific embodiments of the invention describedherein. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the invention maybe practiced otherwise than as specifically described and claimed. Thepresent invention is directed to each individual feature, system,article, material, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,and/or methods, if such features, systems, articles, materials, and/ormethods are not mutually inconsistent, is included within the scope ofthe present invention.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Other elements may optionallybe present other than the elements specifically identified by the“and/or” clause, whether related or unrelated to those elementsspecifically identified unless clearly indicated to the contrary. Thus,as a non-limiting example, a reference to “A and/or B,” when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A without B (optionally including elements other thanB); in another embodiment, to B without A (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to. Only thetransitional phrases “consisting of” and “consisting essentially of”shall be closed or semi-closed transitional phrases, respectively, asset forth in the United States Patent Office Manual of Patent ExaminingProcedures, Section 2111.03.

What is claimed is:
 1. A method, comprising: exposing a fluid mixturecomprising a first gas comprising a first Lewis acid and a second gascomprising a second Lewis acid to one or more electroactive species in areduced state; forming first complexes between an amount of the firstLewis acid and a first portion of the one or more electroactive speciesin the reduced state; forming second complexes between an amount of thesecond Lewis acid and a second portion of the one or more electroactivespecies in the reduced state; and oxidizing at least some of the secondcomplexes, while oxidizing essentially none of the first complexes oroxidizing less than or equal to 70 mole % of the first complexes.
 2. Themethod of claim 1, wherein forming the first complexes is via bondingthe amount of the first Lewis acid to the first portion of the one ormore electroactive species in the reduced state.
 3. The method of claim1, wherein forming the second complexes is via bonding the amount of thesecond Lewis acid to the second portion of the one or more electroactivespecies in the reduced state.
 4. The method of claim 1, wherein theoxidizing at least some of the second complexes comprises dissociatingthe at least some of the second complexes and releasing an amount of thesecond Lewis acid from the second complexes.
 5. The method of claim 4,wherein the step of oxidizing at least some of the second complexes isperformed while oxidizing essentially none of the first complexes oroxidizing less than or equal to 10 mole % of the first complexes.
 6. Themethod of claim 4, wherein the fluid mixture is a gas mixture or aliquid mixture.
 7. The method of claim 4, wherein the exposing is in thepresence of a liquid comprising an electrolyte solution.
 8. The methodof claim 7, wherein the liquid comprising the electrolyte solutioncomprises the one or more electroactive species.
 9. The method of claim4, wherein the step of oxidizing at least some of the second complexescomprises exposing the second complexes to an oxidative potential causedby application of an electrical potential difference across anelectrochemical cell.
 10. The method of claim 9, wherein the step ofoxidizing at least some of the second complexes comprises exposing thesecond complexes to the electrochemical cell.
 11. The method of claim 9,wherein, during at least the step of oxidizing at least some of thesecond complexes, the one or more electroactive species are immobilizedon an electrode of the electrochemical cell.
 12. The method of claim 9,wherein, during at least the step of exposing the fluid mixture to theone or more electroactive species in a reduced state, the one or moreelectroactive species are dissolved in a liquid that is a separate phasethan the fluid mixture.
 13. The method of claim 4, wherein the step ofoxidizing at least some of the second complexes is performed during afirst period of time, and the method further comprises oxidizing atleast some of the first complexes as part of a second oxidizing stepperformed during a second period of time.
 14. The method of claim 13,wherein the step of oxidizing at least some of the first complexescomprises exposing the first complexes to an oxidative potential causedby application of an electrical potential difference across anelectrochemical cell.
 15. The method of claim 13, wherein the step ofoxidizing at least some of the second complexes comprises exposing thesecond complexes to an oxidative potential caused by application of anelectrical potential difference across a first electrochemical cell; andthe step of oxidizing at least some of the first complexes comprisesexposing the first complexes to an oxidative potential caused byapplication of an electrical potential difference across a secondelectrochemical cell.
 16. The method of claim 15, wherein the electricalpotential difference applied across the first electrochemical cell isdifferent than the electrical potential difference applied across thesecond electrochemical cell.
 17. The method of claim 13, wherein theoxidizing at least some of the first complexes comprises dissociatingthe at least some of the first complexes and releasing an amount of thefirst Lewis acid gas from the first complexes.
 18. The method of claim4, wherein the first Lewis acid is chosen from sulfur dioxide (SO₂),sulfur oxides (SO_(x)), nitrogen oxides (NO_(x)), R₂S, carbonyl sulfide(COS), R₃B, boron trifluoride (BF₃), or a combination thereof, whereineach R is independently H, branched or unbranched C₁-C₈ alkyl, aryl,cyclyl, heteroaryl, or heterocyclyl.
 19. The method of claim 18, whereinR₂S is hydrogen sulfide (H₂S).
 20. The method of claim 4, wherein thesecond Lewis acid comprises one or more species chosen from carbondioxide, nitrogen oxides, R₃B, or R₂S, wherein each R is independentlyH, branched or unbranched C₁-C₈ alkyl, aryl, cyclyl, heteroaryl, orheterocyclyl.
 21. The method of claim 4, wherein the second Lewis acidgas is carbon dioxide (CO₂).
 22. The method of claim 4, wherein the oneor more electroactive species comprises one or more organic specieschosen from optionally-substituted quinone, optionally-substitutedthiolate, an optionally-substituted bipyridine, anoptionally-substituted phenazine, and an optionally-substitutedphenothiazine.
 23. An electrochemical apparatus for at least partial gasseparation, comprising: a first electrochemical cell comprising a firstelectrode; a second electrochemical cell comprising a second electrode;an inlet for receiving a fluid mixture; a conduit in fluidiccommunication with the first electrode, the second electrode, and theinlet, wherein the inlet, conduit, and first electrode are arranged suchthat flow of the fluid mixture received by the inlet can expose thefluid mixture to the first electrode; and an intermediate outletpositioned and configured to receive a reaction product produced at thefirst electrode and separated from the fluid mixture; wherein theconduit and the second electrode are arranged such that flow of thefluid mixture at least partially depleted of the reaction productproduced at the first electrode can expose the fluid mixture at leastpartially depleted of the reaction product to the second electrode. 24.The electrochemical apparatus of claim 23, wherein the inlet is influidic communication with a source of the fluid mixture.
 25. Theelectrochemical apparatus of claim 24, wherein the fluid mixturecomprises a first gas comprising a first Lewis acid and/or a firstcomplex formed between a first Lewis acid and one or more electroactivespecies.
 26. The electrochemical apparatus of claim 25, wherein theinlet is also in fluidic communication with a liquid comprising one ormore electroactive species.
 27. The electrochemical apparatus of claim23, wherein at least a portion of one or more electroactive species areimmobilized on the first electrode.
 28. The electrochemical apparatus ofclaim 23, wherein a first portion of one or more electroactive speciesare immobilized on the first electrode, and a second portion of the oneor more electroactive species are immobilized on the second electrode.29. The electrochemical apparatus of claim 25, wherein the fluid mixturefurther comprises a second gas comprising a second Lewis acid and/or asecond complex formed between a second Lewis acid and one or moreelectroactive species.